Has 280K, but had U-joints and carrier bearing replaced in the past. When going down the highway either in 4th or OD, if I let off the gas and then hit it, there's short buzz/vibration and/or a clunk that sounds like it's coming from under the car between the seats, then it smooths out and drives normally.

Could this be a transmission mount at the cross-member? I had an '85 Camaro do a similar thing (the clunk, not the vibration) and that's what it was. I haven't been under it yet to see.

It needs a clutch, so I'll have it out anyway in a few days. Just wondered if this part should be ordered in advance or if I can find one easily.