The driveshaft itself is superduty... only reason it might be a problem is
center bearing that might go bad... but hey, both my cars have 200.000 miles
and I never changed the driveshaft yet.
You are right on track about flex-disc. When bad, the vibration is just
horrid. As far as the rear flex-disc, just inspect it, if you see serious
crack then you have to replace it. My suggestion is loosen the bolt for the
rear one... slip it out on one side... if all the sleeves where bolt goes
through are solid, then it is fine...
However, like you said... if I gone that far, I might as well change it!
Flex-disc will go bad many times before the driveshaft go bad... Since you
took out your exhaust and the shield that covers the driveshaft... inspect
it... tighten up the bolts. If you feel alot of play in the bearing, then it
is probably shot.
No regular shop can replace the u-joint instantly, it needed to be cut out
and reweld a new one in... then it needs to be balanced. It is all precision
work. Rebuilt driveshaft runs $275+ alone. I think that shope in Seattle is
called Driveline... they are reputable business.