1990 Isuzu Truck Transmission in 2nd or 3rd gear will not c

After working in my yard, unloading mulch from my PU, in 4WD, I attempted to back up after moving the transfer case shifter from 4WD to 2WD, to release the 4WD, the transmission appears to be hung in either 2nd or 3rd gear and will not come out no matter how violently I shake the gear shift (5 speed). This has happened before and have always been able to shake the gear shift stick very aggressively and I could then shift thru all gears, the only different is this time it was in 4WD.

Help,