Sounds like you might have a bur on your sun gear. My dads truck did that. We were never sure if it was because he shifted into 4 high at too high of a speed, or when the rear wheels were turning a little, but it gave him trouble for about two years before the transfer case finally just went out when he was backing up the driveway in 4hi.

Does your truck have the auto mode? Try pushing the auto button first, and then the button for 4hi. It helped on my dads truck.

 

No my truck doesn't have the Auto mode. My trucks problem, as I posted in the 2nd post on this thread is the T case actuator is on it's way out.

It periodically locks.. and won't shift unless I tap on it. And once in a while it will send a DIC message Service 4x4. When I unplug it... It will clear.

 

Yea, same problem My dads truck had. I think the fix for his was about 600 bucks.

 

Sounds like a weak actuator motor.
The early S10s had that failure alot. Sometimes it could even be the grease getting thick with age and a simple dis-assemble and regrease would allow them to continue working. Not cost effective for a shop to do but if you have the time/energy might be worth a try.