I think you were close on your diagnosis, but rather than a u-joint
it may be a front pinion bearing that is allowing the shaft to shake
and damage the seal although a severely damaged u-joint might also be
the cause. One thing to check is the pinion shaft nut behind the
yoke of the u-joint. Otherwise I would find a good large truck
repair shop, might be cheaper than a RV shop. Can't help much with
referral sinceI am in Riverside, 40 miles east or Whittier.
> Need Shop in So Cal