4 Wheel Drive/SUVs: ford 1994 f-150 pickup truck, gear lever, clunk

Question
When I go into 4 wheel drive (push button on dash) it's OK until I go up a hill. Then it  goes into neutral (gear lever still in D) and motor revs up but no power to wheels. When I put it back into 2 wheel drive there is a loud CLUNK and shudder then it's OK in 2 wheel. Dealer has tried twice to fix it with lots of new parts but it's still the same.

Answer
Robert, the problem is more than likely in the t case . What happens is the gears and the forks get worn and when you put it on a bind they pop out, been through many t cases here in the shop, so that is more than likely your problem, you might want to take it to someone who knows what they are doing , thanks and let me know if I can help you any further ADAM