Driveline vibration!!! I need help!

well did some more test driving... the vibration seems to smooth out @ 70+ decelerate from that speed and vibration starts...driving in 2nd gear @same speed where vibration is in 3rd gear, no vibration...drive in 2nd @ RPM where 3rd has vibration, no vibration in 2nd...drive car to speed where it vibrates in 3rd then dump it in to neutral keeping engine rpm up, there is no vibration as well in deceleration from that point...the only scenerio I see where there is a vibration is in 3rd gear starting @50-55 and smoothing out @70+...one other thing the vibration is the worse under deceleration from 65 down


oh and the tires were bought new 6-8 mos ago...we assumed we had a wheel that wasnt true so last week I tried new wheels...still have the vibration....if it was a tire/wheel problem it wouldnt matter if the car was in 3rd or second...this vibration only happens in 3rd gear