Check my thinking, vibration at speed

[Earendil]

I think I have a diagnoses on my friends 325ix, but since my own car with the same problem had different symptoms, I'm hesitant. I'm also hesitant because it's my buddies car, not mine.

Here are the symptoms:

1. High frequency vibrations at speed
2. Can be felt starting around 35mph.
3. Frequency has everything to do with vehicle speed
4. Vibration is generally only felt with the gas pedal is depressed.

driveline issue, right? I checked the driveshaft and it has about half to 3/4 this much place. So it looks like the driveshaft is going south.

Here is what gets me. My much worse driveshaft didn't cause vibration until 50+ mph. My vibration was rather constant, regardless of accel or decel circumstances.

My thinking is that when the drive shaft experience force associated with acceleration of the vehicle, that it shifts to an out of balanced position. But, when decelerating, the driveshaft lines back up and the vibration goes away?
Does that make plausible sense? Or is there a different/secondary issue at play here? The guibo looks and feels fine, and there is no center support bearing on this car.

[peytonracer4]

check engine mounts and the half shaft joints for play.

[Earendil]

check engine mounts and the half shaft joints for play.

Son of a... I checked the transmission mounts and half shafts but not the engine mounts

I'll verify these before moving forward with the drive shaft diagnoses.