Under hard accelleration, the drive shaft was flopping around and hitting the floorpan of the car, making a “thump” that sounded like someone was underneath and banging on the underside of the car with a hammer. It's a well-known issue with 1g's and the fix is either replacing the carrier bearings or filling the worn ones with silicone caulking.

I bought new carrier bearings because the silicone thing sounded kinda cheezy. But, after reading the proceedure for replacing the bearings, I decided to give the silicone a try. It's been about 6 months and I've yet to hear any driveshaft thumping since despite some hard launches.

Info on driveshaft thump and how to fix it:

• http://www.geocities.com/qwik93awd/Vfaq/rearendthumpfix.htm
• http://www.plymouthlaser.com/shaft.htm