2000 Pontiac Grand Prix vibration

I assume you bought it from a used car lot. If so, usually they are "AS IS". As for your vibration, it could be a CV axle. (Bent, worn). It could also be in the differential (where both CV axles connect to the transmission) I would try to raise it on a lift and spin the wheels by hand and watch the drive axles for bending. If it's not exactly true and straight, it will vibrate. It may be bent just enough to show up under load. I'd start there.

The CV axles are not expensive. If it's deeper than that, then it can get pretty pricey. It may also be difficult to visually diagnose. You'll need an experienced eye. Hopefully I can help. If you can get both front wheels off the ground, you'll be able to spin the front wheels by hand. Do this and look at the CV axle. If it's bent, you'll notice it. Watch real close and see if there's any 'up and down' movement in the axle. It should be turning true and straight. If you don't notice any, move to the other side and do the same thing. If no movement is noticed, that doesn't mean the axle isn't bad. You may notice it better if you put the car in gear with it running and accelerate. Actually you'll need to have someone do this for you while you watch the axles. PLEASE do this carefully. Use jackstands and set the PARK BRAKE! This is a very unsafe method of checking this. I strongly recommend taking it to a shop where there's a lift and trained technicians.
Anyway, if you don't notice anything wrong with the axles, the problem may be in the differential. There's a 'decelerate' and an 'accelerate' side of the gears in the differential. If there's some teeth broken on the ring gear, it could cause a vibration under acceleration. This is something to consider. Try this. And be very careful if you choose to investigate by yourself.