My '94 M6 with 176k miles can use some new u-joints. Is it worth the trouble to replace them, or should I just order a new AL driveshaft? Does the new DS come loaded with new u-joints?

How about trying to pick up a used AL driveshaft from a salvage yard? What do they go for, and do all 4th gen AL driveshafts fit a '94 M6?

BTW, my stock steel driveshaft vibrates at above 80mph, and especially when I'm on and off the throttle.

 

Just an FYI. I was just at the Lingenfelter site and they're having a special on their 3-1/2" aluminum driveshafts. $299...

 

Pretty good price for an aluminum driveshaft.

 

You can also use a DS off a third gen. Some of them came with alum drive shafts, I forgot the RPO code though. And I read somewhere GM replaced the stock ds on 3rd gens with aluminum ones under warranty if the owner complained about vibrations. So you could get lucky if you look around, some people just offer $25 and walk out with a 1LE aluminum driveshaft. You might want to do a search here or on thirdgen.org if you are interested.