Try this, jack up the rear of the truck buy the frame so there is no weight on the rear leaf springs and the axle is hanging from the leaf springs. Now check if torque tube spline lines-up with transmission, if it lines-up this could be because one of the leaf spring has a taller arch than the other. When taking the body/frame weight off the leaf springs both springs should have the same arch height caused by the weight of the axle pulling them down. If the spline lined-up Check for cracked/broke leaf spring.
The other thing to check is if the rear spring seat shifted on the leaf spring. Check the rear seat bushings too here is
picture with instructions on how to seat the rubber bushings into there normal place. They might be binding.
page 2 Let me know what you find out.
Thanks
