There's no problem at all with self-leveling rear suspension. The 4.8is has air ride height control at all four wheels that tends to be problematic sometimes, but that's an entirely different problem, it's the front suspension on those. Towing 5800lbs self-leveling rear will be invaluable in distributing the load to the front.

Previous gen automatic transmissions are questionable, but not related to towing. You won't have any issues with the manual.