Carefully inspect the center support bearing for tears and rips and the U joint for play. A used driveshaft is usually fine if both of those items check out okay. The center support usually dies because the DS itself is out of balance, and therefore needs reconditioning. Be sure to replace the grease and seal at the diff input flange. Clean the surfaces before mating them up or it'll leak.
You might also call Beyer Motor Werks and see what they'll charge you for a reconditioned one. We use them/I've installed a bunch of themon an array of cars, including E30s, and they don't come back. Actually -- one did: last week I pulled the trans and engine from a 750iL that had a severe DS framerail damage (bent way back by the trans, even). To my surprise, it had a Beyer shaft, and the center support was bent like a %@#!&^. Of course, we didn't install it .
best, whit