Ok, I've had both the gearbox and diff oils changed about 3 or 4 months ago. I also had the drive yoke (which is the coupling betwen the gearbox and driveshaft that I think you may be referring to) replaced 2 months ago, along with a couple of the uni joints.
Basically, at high speed (ie slightly above 110 km/h ;o)) a very firm vibration sets in for about a 20 km/h band. It is not apparent above or below this speed band, and the vibration really feels like it is coming from the gearbox. If you grab the gearstick it is like holding a drill going through brick.
I know this doesn't sound too healthy, and would be interested in getting some thoughts before I do something silly (and expensive) like get the gearbox torn down!
Red ones go faster.....