Transmission causing vibration?

Has anyone had experience with a transmission that caused a vibration felt in the whole car?

I have a 99 Venture.

I've tried wheel balancing, new tires, front brakes and rotors, and still the same thing.

A new mechanic I visited tested it for a few days and swears the front end is good, and it may be the transmission, but doesn't want to just mess with it.

Any one have similar experience?

Thanks.

Wild guess is it could be the pressure control solenoid (PCS) causing erratic line pressure and possible slipping / shuddering during and after shifts. My 02 Saturn L would shudder hard on 2-1 downshifts and buck lightly on 2-3 / 3-4 upshifts until I swapped the PCS. If the vibration goes away when you shift into neutral though, then this would not be the issue.

I've tried revving in neutral and there is no vibration. I suspect it has to do with something that moves when the vehicle is actually moving down the road. And it does not happen during/after shifts. The transmission is actually quite smooth, for now.

I have extended warranty on the vehicle, but its useless if the problem cannot be diagnosed, even by GM.

If it is not vibrating in neutral check motor/tranny mounts and check CV joints

If it is not vibrating in neutral check motor/tranny mounts and check CV jointsOr bearings. Bearings won't usually vibrate though unless they are loaded, which means you can't easily diagnose it with the wheels off the ground.

There are a million things that can cause a vibration on your vehicle.
You can try the change something and see if that fixes it approach or you can find a technician with an EVA (Electronic Vibration Analizer).
This is a tool that measures the frequency and amplitude of a vibration and based on the results, the technician can pinpoint the vibration to the rotating componant that is causing your concern.
You must understand that there are different orders of vibration and a first order vibration is caused by something that is out of balance, the other orders are related to operational characteristics or damage to a rotating componant.
Without doing this, you could spend a lot of money for nothing

I've tried revving in neutral and there is no vibration. I suspect it has to do with something that moves when the vehicle is actually moving down the road. And it does not happen during/after shifts. The transmission is actually quite smooth, for now.

I have extended warranty on the vehicle, but its useless if the problem cannot be diagnosed, even by GM.

I meant to shift into neutral and coast at a speed where you would normally feel the vibration. If you still feel it under those conditions, then it is probably not coming from the clutches.