I bought a used T-case that had a damaged (plastic) speedo drive gear. I found a chewed up steel drive gear inside when I was rebuilding it with 4.7 gears. I don't know what caused the damage but, I had to replace it or keep eating plastic gears.
Unscrew the speedo cable from the T-case, remove the single retainer bolt and pull the speedo gear assy out. Then use a pen light and look inside to see what the steel gear looks like. It's probably damaged and needs to be replaced.
