Larry's right, determine if it is driveline, wheel or engine source vibration.
Also see here:
http://www.peachparts.com/shopforum/diesel-discussion/61086-rear-wheel-drive-rumble.html#post559254
After replacing the driveshaft and driving for a few thousand miles, I believe that I also have a bad rear motor mount. At certain speeds, undulations in the road surface that significantly compress the suspension, will cause a 'driveshaft speed' vibration to momentarily appear. It's hard to tell but it really feels like it is eminating from the shaft tunnel area of the car. Since the driveshaft, flex disks and support bearing assy are all new, I'm guessing that when the suspension is compressed, so is the back motor mount...probably enough to allow the tranny tailpiece to touch the body somewhere.
I have a rear mount in inventory...it will get changed in the near future.