Sounds like your exhaust is rubbing and vibrating against the floor pan. Either it's damaged and loose, or a motor mount has given up the ghost. I would check there.. Also, could be the center support bearing for the driveshaft. Known to make noise when failing. Good luck. BTW, the W124 has 2 motor mounts, or so it would appear..
2000 Dodge Grand Caravan SE
[This message has been edited by jeffsr (edited 09-29-2000).]