I think the output speed sensor on your is in the trans, but, the PCM determines speed with the rear wheel speed sensor?

 

Something like that, I remember looked it up when it originally happened. The CEL makes me think its got something to do with the rear sensor since that's what he fiddled with today.

 

Rear wheel speed sensor is the one that the PCM uses for the speed. When mine went the speedo wouldnt start working untill about 30mph and for some odd reason it wouldnt shift out of 1st gear untill about 4,000 rpm

 

So then based on that, the one that's going is the one in my transmission. I could have sworn a friend of mine with pretty much the same exact truck had a speed sensor go in the transmission, and when he did, the speedometer was acting like dlyter09's, where it wouldn't move until 20-30mph.

 

Its a possibility that the one in the transmission would cause such a situation. But when mine went out I replaced the one on the rear axle and it got rid of all my problems. Not to difficult or expensive to replace so its worth a shot to start there.

 

The output speed sensor in my truck went bad two summers ago. the only thing it really affected was my shifting, I could notice that it wasn't shifting right in the range of rpms that it should have. Its a really easy fix you just need to crawl under the driver side of the truck and grab a 1'' deep well socket. its toward the front of the transmission on top, in a 1 o'clock position. Just disconnect the pigtail and take the socket to it. You'll leak a little bit of tranny fluid, but its very minute. Put in the new sensor connect the pigtail. Disconnect the battery for a few minutes to allow the computer to reset, and you should be good!