1999 ford f-250 superduty no power in 4hi

I have a 1999 Ford F-250 superduty with the triton V10. when i put the truck in 4hi and drive on a flat surface or dirt road it works fine. but when i go up a hill or i am pulling something in 4hi their is no power the wheels will not spin the motor will rev up to about 2000 rpm and the truck will just sit their and not move and the wheels will not spin but if i put it in 4lo the wheels will spin.
also the factory hubs are worn out so the shift on the fly feacher dose not work and i have to manualy lock the hubs to engage 4 wheel drive i am wondering what the problum could be that would give the truck no power in 4hi and if i should swap out the factory hubs for a set of aftermarket ones.  

It's probably because you're doing something that better calls for the truck to be in 4-LO rather than 4-HI.

4-HI would normally be used for general graded dirt roads and snowy/icy/muddy conditions on otherwise good roads.
If you're trying to pull out a rotten tree stump, climb some big hill, or crawl through some rocks, you really should be in 4-LO for that (it's much better for your transmission too).

If you have auto hubs, a swap to standard manual hubs should make an improvement in strength & reliability. Of course the shift-on-the-fly will only function while they are locked in. The hubs would not have anything to do with your lack of power however.  

thanks, the hubs taht are currently on the truck have an auto shift on the fly but that part is not working and is just wore out according the shop that looked at it but i can still lock them the use 4x4 but i was wondering if it would be better for the truck to get rid of the auto hubs and switch to regular lockouts.  

Yea... if you are revving the engine and the wheels aren't spinning, don't. You're just heating up the transmission and I have seen way too many SuperDuty's catch on fire from doing this. It's not a pretty sight.

Out of curiosity, what size tires are you running? Lift? Gears?

Reason I ask is sfr4x4 is probably right, you need to be in Low for what you are trying to do, but the wheels should still be spinning in Hi.  
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the truck is all stock exsept that the tires are 265/75/r16 ATs and not the 253s that ford recamends i have only had the truck for about 4 months. and at the time i had the problum i was going up a sandy hill that was not that steep with a flat bed trailer that only had about 2,000 pounds on it and as i went up the hill i contunued to lose momentum untill i got almost to the top and the truck just stopped. the the motor was revving to around 2,000 rpm and the wheels would not spit even in the soft sand i was in. i put it in 4lo and went the rest of the way up the hill. the reason why i ask is that my old truck whiach was 1995 F-150 would have had no problum going up that hill loaded in 4hi.and a friend of mine has a chevy 1500 and he wanted to have a pull off with me in the sandy we were both in 4hi and his truck just dug down but mine just sat their and would not spin the wheels until i put it in 4lo.