tranny or bad ujoint?

96 tbird lx with 4.6L

when im puttin along at 20,30 mph and the rpm's are just above 1000 there is a vibration throughout the car but when i hit the gas and get up to about 45,50,mph and 1500-2000rpm it goes away, doesnt always happen but usually when im goin light on the gas

This is a tough one. If it was a bad u-joint it would increase with the speed of the car not the engine. It could be anyplace in the drive train. From bad motor or tranny mounts to a bad bearing in the wheels or the differential. I would first check the u joints for tightness and also check the bolts on the yoke to the input shaft on the differential. Jack it up and rotate the wheels to check for bearing roughness. you can check for a bad motor mount by opening the hood and watching the engine. You set the brake and put it it drive or reverse and if the engine raises up under power you have a bad mount.

96 tbird lx with 4.6L
when im puttin along at 20,30 mph and the rpm's are just above 1000 there is a vibration throughout the car but when i hit the gas and get up to about 45,50,mph and 1500-2000rpm it goes away, doesnt always happen but usually when im goin light on the gas

Have you checked the transmission fluid level?

Read over these Common Transmission Problems (http://www.tccoa.com/articles/tranny/transmission/page4.shtml)
to see if any of them might be what you are experiencing.