When Steve McQueen burns rubber in reverse, the rear wheel bounces up and down. Is that a sign that the Mustang has worn shocks on the rear suspension?
I believe that was known as leaf spring “wrap” that caused the wheel hop. The Shelby style “under-ride” traction bars pretty well dampened that problem, but I don’t think the Bullitt car had those mounted.
My fastback had a non-posi 8-inch diff (IIRC) - back then it was always the right side that got the power.
Maybe this ‘68 had something else (installed?).
Normal for regular 9even beefed up) suspensions to let the wheels hop when burning in reverse....they’re designed to utilize that sort of force in the forward direction and the weight transfer from a forward take off helps hold them down....traction bars and other assists can be useful but work better going forward too.