The unit itself might have a problem. However if this is a new installation and has never worked properly, consider the following:
If I am not mistaken, the Gear Vendor's overdrive is mounted on the back of the transmission and effectively lengthens the transmission and shortens the driveline. This will change the angle of the propshaft if not done properly. If the angle of the propshaft is too extreme, the driveline will vibrate at elevated speeds and fail the output seal. In the transmission world, we see failed output seals, broken friction clutch plates and vibration all too often when driveline modifications are done by the inexperienced.
The shorter the propshaft, the more change to driveline characteristics are introduced with a small change in operating angle.
To verify the driveline, an inclinometer is used to measure the angles of the transmission, propshaft, and axle. Measuring tools are used to measure the propshaft length, offsets, etc. A computer program is used to calculate the torsional and inertial accelerations the driveline is producing.
This is not normally something for the amateur. But if you want to take a look at how it's done, you can find dana's driveline program at www2.dana.com/expert. Click on torsional analysis program.
If the unit had been working fine and this problem is new, driveline angularity or phasing is probably not the cause. whew. Good luck.