Doug,
Good catch on the U-joint, but I know of no u-joint that screeches under load, so your symptoms may still exist.
One of the belts is the most likely scenario, is it worse when cold?
The other possible culprit if it is coming from the clutch area is a throwout bearing. If it persists try just riding the clutch enough to take up the pressure on the thowout and see if the noise changes tone or stops.
Cheers,
Julian
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