Transfer case trouble

[721JSD]

Hi everyone. I have a 2001 Chevy 2500HD ext cab s/b 8.1 6spd manual with manual shift transfer case. Here's my problem: When I pull out and start going through the gears, the t-case sounds like its Getn stuck between 2wd and neutral. Makes a grinding noise. Seems to do it when shifting gears or coasting...especially in 5th or 6th gear....when its not under "load". The t-case is full of fluid. It has also done this while climbing a hill in 4low....was fine while I was giving it throttle but as soon as I let off the gas it popped out...."neutral"