How do you know the driveshaft is the problem? Have you taken it to a shop and had it rebalanced? Have you replaced the u joints with the correct ones? A drive shaft is a drive shaft, it dosen't matter if it was from a 6 cylinder originally. There are people who have beat on the 6 cylinder shafts for years and years with built engines and manual transmissions and never had a problem.
very tru but i wanted a lighter one anyways so i figure i get what i wanted anyways and hopefully fix the problem.
i sheared a rear u joint on my 66 impala years ago, split it in half, luckly it didn't drop the driveshaft. drove it 300 miles before i finally found out what was going on. i was driving to little rock ar and finally pulled into a sears and had them replace it. car vibrated alot down the road. oh ya, i lost a little bit of transmission fluid also.
