To bump/update this: I took my 85.5 from the bay area to LA and back not long ago. Odo was working just fine. Got home, parked it for a week or so, then noticed on a drive to work that my odo had stopped. (It's worth noting at this point that I have replaced my previously chewed up odo gear a year ago and have had no problems.)

Had a few minutes to yank my cluster tonight -I'm getting too good at it now- and took a look. Gear teeth are all lining up, and the worm gear turns by hand. Kinda smells like burnt electronics on the little board there, but I don't see any damage like what had happened to Doabarrelroll's up there.

Gonna hafta pop it back in to test if it's a loose connection or something more sinister.

Assuming it's not a cluster-based problem though, is the plastic gear in the tranny actually IN the tranny? would I have to drop it to troubleshoot that?

Thanks for the advice everyone.

Update
 

So I popped my cluster back in as I had to take the '44 northward to deal with some body work.

Odo worked just fine, I'm guessing my problem was a loose connection and or grounding issues.

My odo stopped working, and I replaced the normal gear that breaks, but it stopped again.
For the longest time I thought it was an electrical issue, until I took it all apart to find that one of the other gears was missing a few teeth. Bought a used cluster cheap, pulled the odo and scavenged that gear. All works now. Just another thing to check.