I can’t take all the credit as I read about it in an old Hot Rod or Car Craft tech article. Back when I had an open mind and I actually bought those magazines. Lol
That method is actually in the Mopar FSM's as well for determining if the driveshaft is imbalanced, possibly where the magazine guys got it from? Lol. Those old Chrysler engineers were clever.
One thought I had is the rear leaves could be starting to wear out and allowing too much axle windup (dynamic pinion angle) when on the throttle since OP mentioned it goes away when letting off the throttle.