Are you sure it is in the back of the car you are actually feeling it? Not control arm bushings or alignment that is bad. If it wasn't doing it at the begining of the year and it is now then something is wearing down or broken. I would jack the car up and unload the rear suspension and recheck the diagonal and rod ends on the ladder bars. Also is the diagonal held on by bolt in mounts that attach to the brackets or are they weld on mounts? If they are the bent bolt on type they may be shifting down track creating this problem.
