Had that happen to my SLC. Did a search, got this info, used it, and it worked beautifully. "What typically happens with the odometer is that the trip odometer starts dragging and halts both of them. I don't know if you want to take this approach, but if you don't need the trip odometer to work, you can remove the plastic spur gear that connects the two, and the main odometer will begin working again."
If you want to fix it, here's what's been posted previously:
"Remove the instrument cluster from the car and dis- assemble it till you get to the odometer assby. The problem is the gray metal gear on the number stack. It is supposed to be press fitted to the shaft and spin with the shaft, but over years and age and many miles it loosens up from the shaft and fails to function. Fix it by pushing out the shaft with the correct size drill bit then support the number wheels with the shaft removed. Set the shaft next to the stack and note where the metal gear was positioned. Next take a punch and hammer and make a dimple on the shaft where the gear goes. Press the shaft back through the stack, re-assemble everything and the odometer will now work."
I know you'll be able to fix it,
1980 300SD (Anthrcite Gray) 253,108 miles
1982 300SD (Silver Blue) 242,398 miles
1982 300SD (Black) 263,185 miles