I am buying a '92 Saturn SL with 183,000 miles. It has a loose motor mount, and when we drove it home it shifted real hard, like a bump-shift, and seemed to be confused as to what gear to go into when going down hills. Could this be caused by the loose motor mount, or is the transmission going?
At 183,000 miles...it could be either/or. When a transmission or engine mount becomes loose or breaks, the engine and transmission will flop around under the hood as torque is applied in the right direction.
This can stretch the shifter cable which is routed between the transmission and the center console shift lever, and therefore produce erratic shifting -- just as if you were moving the shifter back and forth, from one gear to another. Some vehicles also use a detent cable linking the throttle assembly to the transmission. The detent cable helps control accurate shift timing, and can also produce similar symptoms when the engine or transmission are not secure.
Your best bet is to road test the vehicle following the replacement of the broken mount, along with a thorough inspection of all cable attachment points for damage. But if you can drive the car while being light on the throttle, without the drivetrain jumping around and cables being stretched out of place, a continued notable shifting irregularity should confirm an unrelated transmission problem. That doesn't mean the transmission is done for, but there may be something other than a broken mount causing the problem.