gee L1ftmech, his vague description of the problem only led us to throw out possible scenarios... but you're right, if an axle was broken the lift would not move at all... (maybe) i've seen some axles broken but still allow the truck to move, it just didn't drive right but was usually accompanied with some bad grinding or knocking sounds when the axle slipped.

Anyhow....
with the broken axle scenario aside....
i think he should still pull it apart and see whats going on in there. Maybe even drain the differential fluid and see if metal is in it, could be a spider gear problem in the carrier assy. Ya never know buddy ;o)

Anyway the part about checking the transmission fluid?
There should be a dipstick in the top of the case just to the right hand side on the top cover.... as far as i know that is the only place to check the fluid level. The recommended types of transmission fluid they say to use are:
Esso Automatic Transmission Fluid, Mobil ATF210, ATF220, or Shell ATF Dextron. Generally Dextron III will work fine.

But without a truck serial number its all just a GUESS as to what we are dealing with!