Coming down a hill tonight (15-20 mph) I put the car in low gear and the car started to shake. Did not do it in any other gear not did it shake in low gear on level ground.

1999 Grand Am SE
2.4L with auto
115,000 miles

what do you mean shaking.....

though I would be carefull down shifting if its acting funny.

Has your trany fluid & filter ever been changed ?
This should be done every 40-50k miles...