Hi Guys.  New member here asking a question that probably has an obvious answer.  I posted this a fews days ago on the 4x4 truck forum, but after 45+ views, no one has replied.  Maybe one or more of you folks will be more venturesome:

I have a 2000 7.3L F350 Crew-Cab 4x4 with hubs that lock from inside the cab with the turn of a selector switch.  I have a steep concrete driveway, about 25 degrees.  The driveway is steep all the way to the garage door, but was intenitonally left as a rough surface for added traction.  The truck climbs the driveway easily in 2WD, so long as I'm going forward up the drive.  When I need to get something up the hill to the garage, I generally select 4x4 low-range, then back it up the hill, put it in park, set the brake, open the garage and load or unload the truck.  Until recently, I've never had a problem with this approach.  

About a year ago, the right front wheel bearing gave out, so I took it to a dealership for repairs (Ouch!...$750 bucks...)  A couple of months ago, I needed to back the truck up the driveway to load some stuff, which I did as described above.  The driveway was wet.  I first had some difficulty with rear-wheel slippage, which kind of surprised me, as I couldn't remember it ever happening before as long as I hade it in 4x4.  After a couple of tries, I just romped on it a bit to get up some speed and made it to the top, where I secured the truck as usual.  I loaded the stuff and went in the house.  When I came out the next morning, the truck was at the bottom of the driveway with two skid marks from the rear tires.  

This happened once more, and I was there to watch it happen.  The front wheels just rolled down the hill while the rear wheels slid.  Surprised the heck out of me, not to mention being dangerous to anyone or anything at the bottom of the drive.  At that point, I began to suspect that the hubs weren't engaging when I selected 4x4 low from the cab.  

Now the questions:  Shouldn't the front wheels have been locked up by the hubs, such that putting the tranny in "park" locks them up just like the rear wheels?  If that is true, is there anything that might have gone wrong at the dealership when the wheel bearings were replaced?  Something that wasn't connected, or was broken, or improperly installed?  

I haven't needed to use 4x4 since the bearing repair (other than the driveway thing) or I might already have the answer...  

well sorry to here that the pup wont stay put...block the tires w/wood, it should stay put ( I know you dont want to here that)...second, punch the guy right in the eye for building tooo steep of a driveway, Ill never buy a house or rent one that I need a running start to get up the driveway.and 3rd, It might be that there is nothing wrong w/your 4wd system and that the weight of the rig facing down is pulling down (gravity) hence the rear tires skid marks! My suggestion would be to load it and turn it around so the weight is on the high side.  

you may try selling the truck and buying a chevy lol................j/k  its prob just the weight regardless its not good to rest the vehicle on the drivetrain any ways chalk tires, turn them side ways etc its prob fun driving up it though  

Thanks for the reply Keithpapa, I hear what you're saying and you're dead right about the weight pulling the rig downhill (in 2WD, for sure) but let me re-state the question: "If you've got 4WD engaged, and you have your transmission in park, aren't the front wheels supposed to be locked?  Should they be able to roll free?"  My sense is that they should NOT roll, that the transfer case has the front and rear differentials locked up solid.  If the front hubs were engaged, then the front wheels should not have turned with the tranny in park...  Am I missing something?  

Yeah, maybe some day...I've always been a little soft in the head for the Chebbies.  

Back to my problem:  I've done this backing up the drive thing (without any problems) maybe fifty times in the past eight years, and it never did this to me before.  Not until after I had it worked on by the dealer!  Do you have an idiot proof way to check that the hubs are engaged, (like driving in circles in a parking lot) without damaging the drivetrain?  

Well, lets think about this for a min...I know that if you lock the hubs in on mannies that the hubs lock w/the inner axle shafts, and in a manual t case the system should be engaged but still moveable w/some tension w/out any power to it...and move freeer w/the t case disengaged. But on the electrical 4x4 mode as in your rig Im not sure if the t case goes to the disengaged mode when you shut the rig off (and theres no power to it) after it was engaged...thus the front wheels rolling when you think they shouldnt.
someone correct me if Im wrong but i think thats how it works.  

jack up the truck so the tires are off the ground and engage 4WD and see if the front wheels spin. that'll tell ya if the shop screwed something up.
Dude I hope you don't live somewhere that gets a lot of ice! You must have a great view from up on the hill. Or is it a defensive thing?
I think keithpapa may be right about the elec. disengaging with power off.  

I think the electronic t-cases disengage when the ignition is off as well, but not 100% sure on that. Chock them tires!  

Well guys, it looks like there might be a problem with the vacuum lines that actuate the hubs...  Problem will soon be solved.

-Thanks  

OK.  Here's the real answer:  The hubs are vacuum operated.  The seals in the left hub are/were leaking, so the hubs won't engage automatically.  It's not the hub the mechs worked on, it's the other one, so (NATURALLY...!) it's setting me back another $300 to get the seals replaced.

Had the hubs been engaged, then the front wheels would NOT have rotated.  The "park" position in the tranny and/or the parking brake would have (and do) effectively lock the front wheels too because the driveshafts and hubs are tying everything together (assuming the t-case is in 4WD and the hubs are engaged.)

The t-case is electronic shift-on-the-fly (ESOF) but electric power is only required to engage or disengage it.  Once it has been engaged, it will remain engaged until the ESOF selector is moved to a different position.  It does not require electrical power to remain engaged, so if you shut off the key, it will still be in 2WD, 4HI or 4LO, depending on which mode it was in when it was shut down.

I'm done with this thread now.  Thanks for responding.  I appreciate your suggestions, especially the one about the Chebbie...    

Well, now we know. I would have guessed they were "electronical" as opposed to "vacuumical". They're kind of "nuc-u-lar"!  

Yeah Fatman I would have guessed the same. when are these manufacturers going to get away from that vacuum crap? Why can't they just go back to the good'ol mechanical shifter it the floor, remember the term "Keep It Simple Stupid". If it ain't broke don't fix it.  

thats why I stay away (or try to anyways)  from 4x4s w/elec shift and buy only the ones w/manuel shift...buts thats good to know about your frog and why it wouldnt stay and how their system really works.  

That's why I bought my '06 Silverado. I saw the manual shifter (which is actually a downgrade from standard equipment) and got all excited. I wish it had a manual transmission too!  

LONG LIVE THE MANUALLY SHIFTED T/CASE