I have had problems with the speedometer for a while. The needle dropping to zero and the transmission shifting erratically but not all the time. This sometimes happened when speed went over 60mph and when I let off the gas and slow down it kicked back in. But like I said, not all the time. Yesterday as I pulled away, the transmission didn't shift right away, then it shifted very hard and the yellow O/D light on the shift lever started flashing. I also noticed the speedometer wasn't working.
Now I have replaced the speed sensor in the rear end so its not that.
Any ideas???? This is really driving me nuts.

 

you fixed what i would have suggested. have you pulled codes yet

 

P0500, thats the code its showing

 

one option: is there a second sensor in the tailshaft of your transmission?

second option: i like this one better since you said your speedo is acting strange is the the PSOM which is in the speedo head itself. it intercepts the signal from the VSS and sends it onto the PCM/computer

 

The OP's 1996 does not have a separate VSS in the transmission tailshaft or in the transfer case. Did you use a meter to verify the VSS signal was making it to the PSOM?

There have been several cases here where the RABS control module was faulty which was pulling down the VSS signal amplitude causing this issue.

Do you have a manual to show you what the tests for the VSS are? As well as where the RABS control module is located?

 

Some diagrams and info from Steve83:

F-series diagram and testing info: F-series diagram

PSOM and info: PSOM picture

RABS info: RABS info