2003 RX Driveline Vibration?

Greetings,

I'm driving a 2003 RX with nearly 108,000 miles on it. A couple of
months ago, it received its 105,000 mile service and all seemed fine.
However, in the past week, when the vehicle speed approaches 35mph, a
severe vibration begins. It doesn't matter what gear I'm in, I can
shift to neutral and it continues. It doesn't increase in severity as
the speed increases. It is NOT coming from tire imbalance since the
steering wheel does not shake, the vibration is distinctly coming from
underneath the vehicle, or so it seems.  Below 35mph it seems fine. My
Lexus dealer is looking at the vehicle on Tuesday. In the meantime,
I've been looking at some used vehicles. Any ideas as to what the
problem is and how much of a $$ hit it is going to be. I can get a
certified used Highlander limited with half the mileage for about the
same value as my vehicle (but I'd have to replace the timing chain on
it soon).

Thanks.

J