I have read all the posts on this but still have not figured why the speedometer reads 50 Mph when you are doing 70 mph and it does fluctuate a little.

I have replaced the sensor on the differential with no change. I also checked the new sensor for any metal filings after I had installed it.
Funny thing is if I disconnect the sensor on the rear differential the speedo is smooth as silk and the cruise will work. Of course the ABS and Brake light are on. Plug it back in and speedo starts acting up. At one point I disconnected rear sensor on diff and two plugs on the tranny and speedometer was still working smoothly. I have also put dielectric grease on the rear sensor and on the two connectors that are attached to the ABS controller.
Does anyone know where speedometer gets it second reading? Like I stated earlier the speedo still works with the rear differential sensor disconnected.
Any help would be Greatly appreciated.

 

Well I figured out what was wrong!
I had the pinion seal replaced about 6-8 months ago. I pulled the differential cover off and found the small set teeth that the sensor picks up on for the speedometer were ground away. A bolt that holds the spider gears(I think) came loose and wedged it self under the ring gear. It actually pushed against the diff cover and almost punched a hole.

Well I called the shop and they are going to fix it with no cost on my end.

 

You should give them some good publicity for fixing the problem to your satisfaction. Customer service like that should be rewarded.