Volvo: vibration, crank pulley, center bolt

Question
QUESTION: I just paid over $800 to have the rear crank seal replaced because of a small
oil leak in my 1988 Volvo 240DL.  Also replaced during the job was a motor
mount and transmission mount. Upon getting the car back, I noticed a
vibration which is very noticeable between 35 and 40mph.  Almost like a
grinding sound, but not quite.  At other speeds there is a very low sound like
that which would come from a base speaker.  
We checked the universal joints and they are fine.  I friend of mine mentioned
that it may be the flex disk, but I've not heard of such a thing.  What do you
think?

ANSWER: Gary,
Hard to say what is at fault without being able to feel the vibration.
Tires out of balance, bent wheel, u joints, center bearing, drive shaft out of balance. Since it occured after the trans was out I would suspect a driveline vibration. Maybe have the shaft balanced.
I do not think your 240 has a flex disc. You can tell if you have  all u joints (3).

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QUESTION: To describe the vibration better, it is like driving the vehicle at 20mph in 3d gear.  I have eliminated, tires, wheels, u joints, 2 of them, as possible suspects.  IS there a way to check the center bearing?  I read something about a crank pulley.  Do you know anything about that?  Thanks

Answer
Gary,
Check the center bearing by removing it and spinning it, smooth turning good, questionable replace it. If the u-joints turn hard in any direction(bind up) then replace them.
You can test the crank pulley by putting a chain wrench on the outer part of the pulley and use a breaker bar on the center bolt. Hold the chain wrench and turn the center bolt if it slips it bad and needs replacement.