You have come to the right place. Yes, synchros can be replaced.The car I am restoring for my daughter would not shift into 3rd or 4th when we got it. I cracked the tranny apart and there was a broken 3rd-4th gear shift fork that was caused by a circlip letting go on the end of the rod the fork rides on (called a striker rod I think.)Anyway, it is fairly easy to rebuild your transmission, but you have to have some heavy duty gear pullers, and a big vise is a necessity to make the job go easier. The transmission comes apart basically in 3 pieces. I replaced the 3rd-4th gear synchro, 3rd-4th gear shift fork, the striker rod, the clip, all the bearings, and all the seals in mine for about $300 in parts. There is a step by step on how to do it in the online z32 service manual. It is posted on several different sites, google it and it should be easy to find. I bought my parts from Courtesy Nissan
http://www.courtesyparts.com/
Piece of cake!