Shaking problems

I've got a 92 Pontiac Bonneville, and I am having problems with it shaking. It only seems to shake when between 2000 and 2500 rpms, and if I put on the accelerator, the shaking stops. Whenever the car is at a constant speed (over 45mph) is when the shaking occurs. Also, when the car is idling or just stopped at a sign or light, it makes noises from the muffler. It will also shake a little when it's idling or stopped. If anyone has any ideas as to what the problems could be, please let me know.

Thanks.

It sounds like wheels need balancing and/or alignment on suspension.

Whats your gas mileage like? Have you checked plug wires, and for a clogged cat converter?