There has been a vibration at about 40 mph that I believed to be in the drive line. It developed over the last few months and will occurwhile at this speed in any gear except neutral.
I was initially convinced that it was in the drive shaft and I replaced the center and rear U joints despite their good condition. I also lubricated the center bearing even though it was replaced 15,000 miles ago. All the rear suspension bushings had been replaced within the last 8 yrs except the pan and where in good shape.
This vibration is not being transmitted through the shifter and seems to be vaguely transmitted through the wagon body.
The transmission mount was replaced 45,000 miles ago and the engine mounts look ok. I still suspected failing mounts until recently when I noticed a similar vibration occurring when engaging the transmission.
This vibration will only occur when cold or when the car has been sitting for more than 30 minutes. I can sit with the car in gear and my foot on the brake, raising the rpm and the car will vibrate, shaking even the muffler system. Within 30 seconds the vibration dissipates and the car runs smoothly except at about 40 mph.
Both vibrations could have been developing concurrently. They certainly feel and sound similar, so I suspect the same source. Being that this vibration also occurs while stopped should rule everything out except the motor and transmission. The timing belt and tensioner were all replaced a few years back. I replaced the transmission oil every 2yrs which is still clean and at the correct level.
All I can do now is wait for it to get worse or for something to break unless someone out there has a clue or has experienced something similar.