RX300 VIBRATES WHEN COLD

I had some extensive (and expensive!) work done on my 2001 RX300
recently.  I had a rebuilt transmission installed, and the timing belt
replaced at the same time.  A Toyota dealer did the work.  Since then,
there is a vibration in the front end that can be felt through the
steering wheel and through my feet on the floor at speeds over 50 mph.
After about 5 minutes on the road however, the vibration goes away.  I
rotated the tires, but that had no effect.  I've never seen a
vibration problem that goes away when the car warms up.  Are there any
Lexus or Toyota techs out there who have come across this?