When my truck had a failing axle u-joint (the needles all came out and it was all rusty),the truck started doing a weird "pull"to one side then dart the other way rapidly,usually after taking a corner and accelerating normally..even in 2wd with the hubs not locked..
At first I thought it was loose drag link ends,replaced those,they were a tad sloppy,but it made hardly and difference--I checked the frame for cracks at the steering box,nothing wrong there,all the bolts were tight too,no loose rivets on the front crossmembers either..steering box was OK as far as excess free play or "tight spots"..
To me it felt like there was someone at the other end of the steering colum trying to steer in the opposite direction as I took the corner,then it would swerve back into the lane..
It only did this once in a while,but was un-nerving enough to demand immediate rectification..I didn't want to end up bouncing off an oncoming vehicle or hitting one head on..
After I decided not to risk plowing a whole winter with a U-joint ready to pop,I decided to replace it...and the truck has not done that weird steering BS ever since..its possible the wheel bearings on the same side were a tad too loose too..