I researched this here on this forum and found these guys highly recommended:
http://www.driveshaftspecialist.com/Alu ... 0Alum.html
I called and spoke to a very helpful and knowledgeable guy there a couple months ago, he said he had the measurements to do a custom one piece driveshaft but also showed a stock replacement 2-piece being available - both around $700.
I'm not sure what the advantages of a 2-piece driveshaft are. Obviously a one-piece can be lighter and have less parts to fail in the future, so why do so many manufacturers use a 2-piece design? My '90 is going to need a replacement driveshaft before too long as well, so I'm going to have to make that decision soon.