Without a WD hitch you would feel pronounced motion of the TT levering the back of the truck up and down as you go over bumps and expansion joints. A properly set up WD hitch will greatly attenuate, but not completely eliminate, this sensation. I'm guessing that is what you are feeling. But if you're feeling the tail wagging the dog or notice the trailer moving back and forth behind you, the hitch may not be set up properly (instructions can be downloaded from Progress Mfg's website and you can check with a measuring tape).
I usually set the controller so the TT brakes, when activated alone at slow speed on pavement, will slowly bring the rig to a halt. If I am on gravel in a campground and the TT brakes lock up, I will temporarily reduce the setting. The times you're on gravel, you're not going fast so stopping help from the TT isn't as critical. That's why I generally set it for pavement, which is what I'm driving on 99% of the time.
Air up rear tires to max sidewall rated PSI. Same thing with TT tires.
Backing up, you could conceivably crimp and damage the A frame of the TT with your rear bumper. Backing up sharply is fine, up to that point where you might hit. With a pickup's big turn radius steering, sometimes the truck cannot pivot as fast as the TT to straighten out, and the angle will increase as you continue backing up; watch out for this. If you bend your A frame you won't be a happy camper.