I had to replace a speedo cable and the gears in an old 84 4x4 RC I had. Since there was no cruise control, it was simple to replace the cable. One piece from the back of the instument cluster to the back of the transfer case-that's where it goes in.
The guy I had purchased the RC from told me that the speedo didn't work and he was quoted a lot of money to get it fixed. In addition to the new cable, I had to replace the pinion gear and the sleeve that is inside the back part of the transfer case. They are both made of plastic and for some reason they had both been ground smooth. I'm not sure why, but after I replaced everything I never had a problem.
As far as the gears, Here's some pics from an 84 fsm. I bought my new pinion gear from a dealer and I told him what size tires I was running and he gave me a pinion with the correct tooth count.
To replace the sleeve, I just disconnected the rear driveshaft at the t-case and took the back cover off the t-case. The sleeve slid right off and the new one slid right on. It was that easy.
I don't remember how much everything cost, but I do remember being surprised at how cheap it actually was.
Also, this was one of the first things I ever did on a vehicle besides change the oil. It was pretty simple.