RB it is a pretty simple system. You have already replaced a third of it. The other pieces are the actual speedo in your cluster and the gear in your transfer case. if you pull your cable off the TC and put a drill on the inner (spinning) part you should be able to spin it and it doesn't have to spin very fast to get a reading on your speed-o-meter. If you getting a reading when you do that then the problem lays in the speedo gear in the TC. Remove it and the culprit may be a worn gear etc... I f it is the meter its self I would look for a replacement however someone here may know a way to repair them, that's not me.
On my 71 FJ40 you have a speed maker that connects between your speedo cable and transfer case. If it's broken your speedo won't work
They now make a one piece speedo cable that goes directly from the speedo to the transfer case and bypassing the speed maker.