5kcstq drivetrain noise/vibration

[EKellock@aol.com]

Since the new tires (D60A2), I can now hear/feel this much more clearly (them
old tires was cupped).  It's like something in the drivetrain is out of
balance or a mount is bad and transmitting engine vibration through the car.
 My mechanic rode with me and said he didn't hear anything that he would be
concerned about, but you know how that goes... wait till it gets really bad
so they can actually find it. 

It does not vary in relation to turns and it goes away completely if I coast
with the clutch in.  Coasting in gear with my foot off the gas I *think* I
can sense something.  It's more pronounced in 5th than 4th, but definitely
present in either gear.  That's what got me thinking about motor or
transmission mounts.  The vibration/noise changes but is still present which
could be a function of the different level of engine vibration at different
rpm's.  At low speeds it is not present; it becomes noticeable at highway
speeds, from 45 - 70.  At a steady state 65, it sounds like a train rumbling
along off in the distance.

Any ideas out there?  It didn't do this when I bought it in May '95.
Do motor or transmission mounts go bad on these?
Can driveshafts get out of balance?
Thanks in advance.

Ed Kellock
Lansing, IA
87 5KCSTQ
87 CGT