I am questioning the transmission also, if there is anything in there that spins fast enough in reverse to shake the hell out of my mirrors. They shake both forward and reverse, just much more noticeable in reverse. I don't know the internals of an 18 speed transmission, but is there any relation to 18th gear and reverse. These two holes seem to show the most symptoms. The trans is a new reman six months ago. I am grabbing for straws now. Clutch or Trans.
I also have a hop that is driving me nuts around 61 to 70 mph is when it seems to be the worst. Comes and goes at times. I don't know if these two things are related, or I just have two issues.