The fact the lever effort has changed sounds suspicious. I assume when you say the "shaft locks up" you are saying when you move the lever, you can see the shaft that enters the trans case move properly?
The drive shaft has couplers on each end. Don't know if the NH's have issues with them but I can remember several TBN members losing FWD due to drive shaft couplers or pins failing on different makes.
If you can see the lever linkage is all working properly, I would check the drive shaft. I would probably also lift the front wheels off the ground, rotate them and confirm the drive shaft is rotating with them. Do all this an you have pretty well isolated the issue to inside the trans case. Not familiar with your tractor but usually you can take the top trans cover off and visually see most areas of the transmission like the FWD engagement shaft, fork and gears. This would help greatly in diagnosing the issue after everything else is ruled out.
