are you referring to an obvious looking gap between bumper and tailgate? or does that side of the car sit lower? maybe a sagging shock or coil spring?
The bumper is at the top of the slot on both sides. If i lowered the other side to match it would look ugly i feel, as both sides have a pretty big gap.The bumper mounting plates are slotted for adjustment. Once the “impact absorbers” are mounted to the frame you can adjust the bumper itself up, down, side to side. Shims can be uses also to set the body to bumper gap.
First get on level ground. Check that the frame is level. Make sure tire size and pressure match and the body is level to the frame. This can also be shimmed to get things aligned. Finally set the bumper spacing and adjust as needed. Once locked in you can R&R the bumper at that 3 bolts to the frame flange and the 1 bolt at the end of the absorber.