I have done this myself on the same speedo/odo.

I don't recall the exact procedures, but it was a relatively easy, one-evening fix.

Pardon the vagueness and lack of details, but it's been a year or so.

I recall that the dash pod pulls out toward the steering wheel. I was able to disconnect all of the light plugs and the vacuum line by twisting the pod to odd angles. The speedo cable was a challenge. I recall going in just above the brake pedal and loosening the cable in there. THis gave just enough slack to pull the dash pod out a little more. The cable then unscrews.

R&R was logical, straightforward. Pay attention to the possibility of the thing falling apart.

My situation created a loss of friction between a pot-metal drive gear and its steel shaft.

Pulling the shaft out from the gear, I crimped it slightly with a vise grip to give it some texture. A gentle rap or two re-seated the shaft, and I've been back in business for a while...

The job was disconcertingly easy, as if a bad guy hooked up a Dremel to that shaft, he could have spun back 100K in several seconds without disassembly. Not good.

Good luck...