Interesting low speed vibration, 5KQ and 5KTQ

[john at westcoastgarage.net]

A new one to me, 86 5KTQ has a low speed vibration, feels like a 
driveshaft U-joint when leaving from a stop.  Throttle position isn't a 
factor, only speed.  After about 20 MPH, the vibration is gone. Doesn't 
matter whether you start out in first or second gear. I thought maybe 
we'd disturbed something when we did the clutch, but a phone call from 
another customer yesterday is making me rethink that conclusion.  In his 
87 5KQ, the vibration appeared 7 years after the clutch job, leading me 
to believe it may not be related.  The driveshaft guy says it's not a 
normal symptom of a driveshaft failure, but these aren't something with 
which they deal with any real frequency.  Any hints/questions/BTDTs out 
there that might help me solve this problem?  Lotsa miles on both cars, BTW.

TIA, John