3.09 gear set with 3.23's

If it changes with the throttle,or power application; it may have something to do with the pinion angle or the springs.
What I did on one job, was install an adjustable snubber and jacked it high enough to force the pinion into a more or less a fixed angle, then road tested it. On that job it turned out to be a bad right-side spring. A new pair with new bushings cured it. Sometimes I get lucky.
On another job, I ran the combo into the vibration zone, and noted the rpm and engine vacuum. Then checked the timing. Then changed the timing a bunch and retested. That cured it. So I figured the engine was vibrating and sending it thru the clutch all the way to the back and into the springs. But it was not perceptible in neutral. So I figured the engine had to be under power. I'm drawing a blank on what it took to cure that one, but it most definitely was in the engine tune.
The Mopar unibodies have a natural resonating frequency at about 50/60 hertz in the A's. With the right gearing and speed the tires can excite this frequency, and send a somewhat annoying whoo-whoo-whoo thru the car.. Mine, with 3.55s, starts at about 50mph peaks at 55 and is gone before 60; So I can't drive 55, lol. I found mine by blasting up to 70, putting the trans in neutral, and shutting off the engine; then coasting down thru the vibration zone, and Hyup, there it was. This vibration varies only a little with gear, tire, or power changes.