New shocks for '06 GMC Sierra 1500HD?? (Rear end bounce)

I have not weighed things yet, but my front tires sit at 35 7/8 (ground to top of wheel well) without anything hitched. When I add the TT without any WD equipment, front rises to about 36 1/2. With WD I go to 36 1/8. So I am about 1/4" diff between nothing hitched and WD installed with TT.

Again, the bounce occurred with a boat too behind the truck. Just feels like the rear is soft. However, when I "rock" the rear up and down, then stop, it doesn't keep bouncing but stops right away.