The U-joints are replaceable, they come from a company called Rockford, easiest availibility NAPA.
They can be trickey to replace though. There are two things to consider: 1) the driveshaft is a balanced assembly, you must mark the segments so it can be assembled in the same relationship as it came in, or be prepared to pay to balance, 2) the yokes are, well, weak, they will not stand a lot of pounding, or they will collapse/bend.
What I do is use a cutter disk in a grinder to slice out the center of the old joint (after marking the driveshaft and yokes so I can re-assemble it the way it was), then I push the caps towards the center with a socket and a "C" clamp, then I clean up the holes (where your new caps will go) with a Dremel tool (mini grinder), then I carefully re-assemble the driveshaft using a "C" clamp, being real careful not to distort the trunions.
http://www.rockforddriveline.com/replacem.htm