That is what I was going to suggest also. We just had one go out. One of these posts talked about sometimes they start vibrating when they are going bad. I would suppose it lets the drive shaft get slightly out of center as the rubber gets bad.

Quite a bit of maintenance rubber parts start going bad on the 9000 series as they get 8-10 years old or 4000-5000 hours. Engine crankshaft dampener, engine to transmission drive line dampener, cab mounts, transmission mounts and the rubber pads on the steering cylinder pins that I can think of.

John