cridom

I have a 94z with 52k miles and M6, my car has been sounding very odd with these shaky sounds and vibration so i thought it was the trans mount or exhaust that i put on. i took it to meineki and the pipe that goes to the pipe had an exhaust leak and the bolts on the cat were gone so they welded everything and a lot of the vibration is gone , i think the exhaust was amplifying the trans vivbration. anyways when i started it and put reverse i heard a clicking sound u think this is from the rear calipers?
also how hard is to change the trans mount?
thanks a lot