/// Start with your motor mounts. Some how the change in torque is shifting some part of the engine/transmission to place an out-of-position vibration in the drive shaft.

Volvos are noted for this type of vibration in the drive shaft but it usually involves the engine/transmission nose diving on hard braking. It is usually the motor mounts but as the other posts said it could be any combination of mounts, center donut, etc.

A severely worn rear bushing could concievabley be involved but it would be a second choice. Those bushings would usually be involved in acceleration vibration.

BTW where do you feel the vibration, steering wheel, seat, or rear floor when a hand is placed on it?

Have you had the propshaft UJ's checked, as well as the centre bearing on the prop?

Regards

Low idle speed? You don't mention model but idle speed most likely should be 750 RPM in gear or in nuetral/park and higher at 900 rpm with AC on.

Sounds diveline-driveshaft related. Perhaps the drive line in out of sync, i.e., the U joints are not properly lined up. Also could be a problem with the center drive line mounting. Check to see if mounts are secured to crossmember, front mounts are ok and that rear end bushings are ok.

This is one of those things you have to check out nearly everything involved with torque changes.

dick