Wobble at 40MPH
 

2001 Buick Regal GS supercharged, 80k miles
At 35 to 40mph the car wobbles, "only under a load" traveling downhill with no load it's fine. It's not RPM dependant, under a slight load it will shake at 35 to 40 in second gear or overdrive. No unusual sounds from cv joints. I have rotated the tires, looked at cv joints, lower motor mounts look a little weak. This wobble is felt thoughout the car.
I have not taken it to a dealer yet as I have spoken with a few and they seem confused.
Thanks so much for any help
PS the engine seems to be very smooth and doesn't miss a beat.

I have a similair problem but no body on this forum seems to know anything about Buicks. I've asked several times to no prevail.