Bad tranny mount symptoms?

How would you know if your tranny mount is bad? I'm getting ready to sell my van and want to make sure she's top notch. I have the smaller 170ci and when I reverse at slow speed it shakes (especially going up an incline) but if I reverse at higher speed it's fine. Forward is nice and smooth.

Thanks
Mr. D

put a jack under the trans and see it moves up before the body starts going up,,,,,,also try to pull the tailshaft side to side,,,,,,,, do the same for the engine,,,try and jack it up,,,,,,,,,see if those 2 mounts hold the engine to the crossmember and are not broken,,,,,,,