Ed, the info Barry has given you is good and so is Larry's. You seem to be getting some driveline windup that is not being relieved. As I read your post and your personal profile, I noticed that no loader was mentioned. If you don't have a loader, the best way to relieve the tension is to jack up the front axle as Larry suggested. This is normally not needed, but if you don't use 4WD much, yours might be stiff. You might check the fluid in your SensiTrack clutch as an additional precaution. Were you in on demand (automatic) 4WD, or full-time 4WD? On demand 4WD allows the sensitrack clutch to engage/disengage 4WD, but full-time 4WD locks up the clutch. This is the position that the manuals say may need the tractor to be put in reverse to relieve the tension. Have you tried the full-time 2WD on the platform just to the left of the seat? On your tractor, this lever engages the transfer gear while the large lever on the fender engages automatic (on demand) and full-time 4WD. It was unclear to me from your description whether you were using both levers or just the one on the fender.
BTW: Be careful about putting too much force on the levers. They can and do bend pretty easy. If they bend, then the slot in the fender may be out of position for the shifting. You could end up moving the lever to the end of the slot, but because it is bent, the actual movement of the shifter in the transmission will not be fully engaged/disengaged.