My car has a bouncing vibration when I have it in reverse or 1st gear. I thought it was doing it when I was letting off the clutch so I figured the clutch had a problem but I raised the car and started to put it in gear with the parking break on but it didn't do the vibration. then I held the rpm's up alittle with the clutch fully engaged and applied the brakes and at a certain point it starts bouncing/vibrating.I tried to mimic as if I was driving the car with tenstion on the engine,trans and rear end. I had some one hold that 1500 rpm range and i looked at the engine but it was only moving alittle bit but the rear of the trans and shifter was bouncing. I looked at the trans mount bushing and it seemed fine but the whole trans crossmember was bouncing. the bolts are tight on it. this whole time the clutch is fully engaged and in gear. this problem is exactly the same when I drive the car. What could be the problem. the trans shifts great and the engine and rear end is great also.

 

Try raising the trans with a floor jack. If the trans mount is cracked, it will lift right off the cross-member. Also look at the torque arm bushing.

 

^^^ What he said. I had a similar problem. When I checked the bushings and mounts visually they seemed fine. After having the car up on a lift and a friend going threw the gears I could see the mount and TA bushing were fatigued. Replaced both and no more issues

 

I just set the jack under the tranny and tried to raise it. It started lifting the car so I was still good. I was told the posi may also give these symptoms.