It's good to hear ( sort of ) that others are having the same problem. The feel is torsional vibe not clutch chatter. In the old days ( mid 70's ) when Chrysler first came out with lock up TC there was much talk of the vibration being TC clutch slippage. It was really torsional vibe from the TC locking up too soon and then engine lugging.

If the TC would not lockup / trans would not hit 5 th at 40 MPH , 1,200 to 1,500 RPM the vibe would be gone. I "think" the TC clutch is modulated so maybe it it is suffering from clutch drag, it could be applying more than requested.

I need to put a duty cycle meter on the TC solenoid and have a look, will probably add a TC bypass as well to test. When slowing and the trans automatically down shifts from 3 to 2 to 1, there is enough of a bump that makes me think the TC is still engaged / dragging. ( feels like a standard shift without any TC cushioning. ) Car does not cold stall, feel like it is tugging at a stop.

I drive it as is but this is to be Moms car and 40 MPH light throttle is right in the around town range. It may come to having her drive it in 4th.