Ah...Sumosprings...
Assume a live axle, right? (solid axle)
If the Sumosprings lift the rear of the truck just a tad...it has unloaded the leaf springs a tad...the leaf bushings need to be pre-loaded in order to NOT have play (compliance) in controlling the rear axle. On that, the leading leaf eye needs to be LOWER than the trailing leaf eye...if not, it will NOT follow the front axle steering input well...
My guess is that there is some movement allowed in the rear axle assembly somewhere...
Before spending tens of thousands of bucks on a different TV...consider adding a PanHard rod/bar to the rear axle. A good suspension shop will know what/how/etc to do. That will solve or address most of any side to side movement of the rear axle assembly