Rear Bumper loose?

If that's a stamped square hole there should be a stabilizer bracket behind it. Repeats on other end/side.
A 67,8,9 B'cuda has 4 stabilizers total. These bumpers aren't significatly wider than there main mounting brackets so they don't wobble badly without the end stabilizers. The turned ends will move pretty good though.
A 6 inch wide bumper on 2 inch wide brackets? I can imagine it wobbles pretty good all over without end stabilizers.