90 F-350 Speedo problems
 

So I have a 1990 Ford F350 and I have searched all over the place and I can't find an answer can someone help me.

My speedometer doesn't work so I have replaced the cable speedsensor and the little plastic gear that goes on the end of the speed sensor. You can spin the gear when it is out of the transfer case and the speedometer works as soon as I stick it in the transfer case it quits working. To fix this am I going to have to tear apart the tailshaft on the transfer case?

So I figured out my problem it is in the tailshaft looks like I'm going to tear it apart and see if I can figure it out.