Yeah, if it's your 30SEL then the exhaust pipe could be hitting the cross member just behind the idler arm. If so, it will do it worse when you turn left and accelerate.
Never heard of shimming up the rear mount, but it seems kinda red-neck logical if the mount has sagged.
If the new mount has brought the height back up and you still have the vibration, it might be the center driveshaft bearing support, or the front flex disk; might have been getting old, and you finished it off when you worked on the rear mount? that center bearing is fun, you get to get up close and personal with your exhaust system.

DG