I started out with the original front differential that had no diff lock, but looking at it a bit closer, I could see that the casting was prepared for a diff lock, all that had to be done was to bore out the housing on a milling machine, drill and tap the hole that holds the electrical switch and tap the two holes that hold the actuator on.
The diff carrier, fork, push rod and guide from a Syncro transmission will fit in there without modification, so I put the original ring gear onto a locking diff carrier that I still had from a core tranny, put new bearings in and set bearing pre-load and gear backlash.
I also took the Viscous Coupling out, there was quite a bit of wear on the splines and I had noticed that the RH side front tire had a lot of cupping.
When it gets older, the VC gets hard, heats up and eventually fails, so I took the VC apart, cut the splines out of it and pressed these into a sleeve. Drilled 4 set screw in each end to prevent it from slipping and voila, a coupler.
Now without a VC you need a decoupler in the tranny, so I will cover this in a next post.