I have yet to come across a vibration free driveshaft in a fox. I've had multiple steel ones and aluminum ones in many Fox's, MN12's and P/U's and all have vibrated to some degree. That being said....if the vibration is minimal, then I would concern yourself with replacing the driveshaft. By minimal, I mean you should be able to bring the car up to highway speeds and not hear feel anything excessive. I've always gauged it by being able to see clearly in my rear view mirror. If the image in the mirror becomes "blurred" because of vibration under 140km/h, that's an indication there may be a problem.
I suspect that it's your pinion flange that has gone south, not your driveshaft. I've had u-joints so badly ceased they would shake my rear view mirror out of adjustment at highway speeds....yet still would not work the driveshaft bolts loose. Red Loctite requires enormous force and many times heat to break the bond. No amount of shaking is going to loosen those bolts....especially continuously the way they have been.
I suspect that they've worked themselves loose in the past and gone untightened for prolonged periods of time and wore the threads of the pinion flange. Try threading the bolts in by hand to see how tight/loose they feel. If there's any play in them at all, that's likely your problem.
If it's been vibrating badly for any length of time, it may very well have taken out your tail stock bushing as well. Check for excessive play, wear on the yolk where it enters the transmission, or leakage at the seal.
