Ok, maybe I misunderstood. what did you mean by "... during acceleration, the engine spools up like it's in 4wd"? I had assumed that the engine got loud, RPMs shot up but truck didn't move correspondingly. A classic sign of low ATF level. Could be something worse if the transmission is headed south, and has a sticky clutch pack or solenoid in the transmission that could cause those symptoms. I wouldn't spend a whole lot of time worrying about the 4wd transfer case as there's really nothing in there that will slip - It's all metal gears and chains. If it's in gear the tuck will move, if it's in neutral it won't. If it's somewhere in between it'll grind. - you know the sound.

Check the ATF level and hope it's a quart low. ATF flush probably wouldn't hurt but may not help if the trans is very far gone...

-Saturday