Ok - here's the scoop - tell me if my thinking is correct:

Vibration in 3rd gear at about 50mph and more, shift to 4th and it gets way less, but is still there.

Unless I'm wrong - and tell me if I am - driveshaft speed is 100% related to wheel speed, which means if I have a vibration at Xmph in one gear, but not any other gears at the same speed, then the vibration has to be in the trans since the driveshaft and wheels do not change speed when shifting gears.

Sometimes I have a very slight vibration in all gears. Sometimes I have no vibration in ANY gear.

Seems to me that the trans is trying to go out?? Maybe the mount is bad??

All helpful comments are appreciated (gotta do a road trip for work next week.)

Thanks.