When the front drive is engaged, the natural power wheel is the left side. That could explain away the negation of your drive issue versus 2wd. John suggestion for the 'clock' check is great. Doing this on my Raider several years ago revealed a broken UCA mounting bolt on one side and a bad lower ball joint on the other.

If you know of worn steering parts, than the time has come to change them for the safety of you, the truck, and the general public...if you continue to use the vehicle.