Can someone please explain to me what a transmission syncro is? About how much would it cost to have one replaced/rebuilt?

 

T56 Syncronizer are made up several parts.
A sleeve, hub, 3 keys and springs. Blocker ring is pushed into the hub ( The hubs alignment is maintained VS the sleeve) by the 3 keys. Picture 5 gear and 6 gear on a shaft. between them is the sycronizer and on each side on the sycronizer the blocker. Moving the syncro pushes the blocker into the hub selecting that gear. Guy's out here alway's say "syncro's are bad" when IMO they have grinding due to blocker ring problems. The sycro is a tough piece compared to the blocker. You can get all the internal hardware you need to rebuild for $300-$400 dollars.