Rear tire ware

Not a crazy amount, but I think maybe the picture was deceiving, the outer edge is pretty good still

 

Get a , zip tie a couple of Home Depot L-brackets to it like I did:



Park on a level surface, place the L-bracket flanges against flat spots near the rim lip, and measure the angle of the wheel. That's the wheel's camber. It should be 89-91 deg, ideally 90 deg.

 

Take your truck to a quality tire shop and have them determine what the problem is. Most responses so far are guesses and dubious opinions.