High speed vibes can come from a variety of sources. The faster it spinds, the more effect a small issue has and th bigger the shake. Stress goes up as a square of RPM. So, if the trans was replaced, and the mount was not right, or good, or is plain rubber as opposed to Urethane, or the engien mounts were pried into place and the engine doesnt sit right, or the engine sits too high as opposed to the trans, or the rear has sagged, or the pinion is off, or the driveshaft is close, but not quite long enough, or the U joints were replaced, but not centered after they were hammered in, you will have a vibration. If it's the usual thrumming sound that seems to make the whole car shake, but that goes away totally at lower speeds, my moneys on Ujoint or pinion angle issue. How much of the slip yoke is showing out the back of the trans with the car on it's tires? Where is the pinion snubber in relation to the floor pan with the car on it's tires?
