Recently bought an old 78 GMC K15 and I hear all these rumors about them,

1. Leaving the front hubs locked you can shift in and out of 4 Hi on the fly even at freeway speeds.

2. You'll blow up your t-case if you drive in 4X4 on dry pavement.  

3. You kill it driving over 55 in 4X4  

4. You wont brake anything leaving the front hubs locked and driving around (even at 55+) with the front hubs locked while in 2wd.  

I have a limited amount of experience with these things and I'm not brave enough to try them out on my own. Anyone know if these are true or false? I'm sure some of it depends on what t-case you have. I think I've got an NP205 and I know I have Warn manual front hubs.

Thanks
JK  

the best thing to do is when you need four wheel drive lock in the hubs and shift t-case into 4hi or 4low depending on what you need it for.when your done unlock the hubs and shift t-case into 2 wheel drive. otherwise your asking for trouble and those parts sure are expensive. #1-that year truck was not designed to be shift on the fly so that means stop first! #2-it's not a good idea to drive a selectable hub locked in on dry pavement. #3-you can drive your truck at speeds in 4 wheel drive but dont be rediculus. #4-you can drive it with the t-case in 4hi with the hubs unlocked as far as i know.  

the k15 is unlikely to have a 205, it should be a 203 t-case, in which case it was designed to be full time 4 wheel drive, having manual locking hubs it's a good bet that it has a conversion kit installed,
shifting on the fly will not hurt it one bit as long as the rpm's match the road speed, not matching the road speed will result in gear jamming, actually leaving the hubs locked in also hurts nothing, it defeats the purpose of having manual locking hubs because the axles and drive shaft will be turning.  

well if it's a 203 sure...  

If it is a 203 that has been converted, you cannot "shift on the fly" unless front hubs are locked because they are no synchronizers in that T-case. Same with a 205, no synchronizers either. They were not "officially" shift on the fly until 81 when they started using 208's. (later models have syncros too)  

when shifting with no syncro's road speed must match the rpm's and it will fall into gear at any speed with no grinding or dammage of any kind, simmiliar to a nascar tranny and a truck trannies, no sync's, just driver skill. sycro's are for people who do not understand how to shift other than shove down a clutch and jam in a gear

matching road speed and rpm is the key shifting even with syncro's, thats why some 4spds are seemingly hard and slow shifting.  

Nice "spin" but the addition of syncros to Tcase was a wise move if you want true shift on fly ability. Myself I rarely shift on fly but when you do it does not make you less talented because you favor syncros vs double clutching and such. Push-n-Pray 4x4 drive control was truly made for less talented and able and for increased marketability too. Syncros were kinda long overdue.  

thats not what I meant there snoman, point was they can be shifted without harm.....thats the reason for syncro's,  

Manual trannies did not have syncros in most of them 50 and 60 years ago and many HD trannies in SUV's did not have a syncro for first until about 15 years ago. Funny how Tcases finally got them and before all SUV trannies did on first gear.  

honestly dude i dont kno that much about the year 71 gmc but i can tell u that its always better to play it safe and just when u go muddin then get out and shift the locking hubs and then shift to 4 wheel cause its always gonna be a safer bet if we dont kno the exact truth u kno and as for the 55+ thing dude u can do what ever speed u want in 4 hi i mean come on that wouldnt make since if u couldnt  

I makes perfect sense, if the rpm matches road speed it shift as if it were parked.....that is how it works with no sync's  the syncros act as brakes and alignment tools all in one allowing the shift to be made by a "gear Jammer"
I think the reason the t-cases finally got syncs is to allow the push-button on the fly cases to work without destroying themselves, and operators that don't understand why they grind.  

I do not believe that they have syncros between high range and low range though and that is where most of the grinding takes place with "Push-n-Pray". Also syncro's generally (in case of Tcases) bring front output shaft up to speed not slow it down as another poster suggested. In a tranny the term "brake" does not really apply because the can speed up or slow down gears until speeds match while a "brake" suggests that it only stops them.  

well i would never shift between hi and lo range while i was moving. i get what mud is saying if you know your rig well enough you can manipulate throttle to adjust gear speed so there is the illusion of syncros.  

why would anybody try and shift their 4x4 in 4wd at 55+ mph your asking for something to go wrong...dont do it!
get out, lock your hubs, get in and shift.
you wont hurt it driving in 2wd w/hubs locked but your mpg will suffer bad
no you shouldnt drive in 4wd on dry pavement,you dont need 4wd on dry pavement!
driving in 4wd over 55 wont kill your truck i do it w/mine at times and it dont hurt it (i dont keep the speed at 55+ either, most of the time it slower for winter reasons!
on my 93 k1500 i can shift into 4wd while driving but its at speeds 35mph and slower not 55+,but most of the time i stop and let everything do what its supposed to do.
if you can help it you dont want too much unessesary strain on any 4wd part  

why wear your wagon out on purpose? all your gonna do is bind up your 4 drive and not be abl to shift out of it when next time you stop cause of funny smell from under said truck,now try turn the hubs ,no not with you bare hands,they got hot from all the innereds and outer...rds turning. try to back up into traffic to unbind the bound up aformentioned innerds etc,ohh and all the grindey sounds you heard as you rammed the t-case into 4 hi? weel thats all the gears trying ta mesh, ...maybe...