One day on the highway my speedo would start jumping around after it hit the 50mph mark and would wrap all the way around and back to zero. Its as if it had a mind of its own. I asked a fellow Jeeper and he said its an XJ thing and when they get old they will do that. So I picked up an instrument cluster at the local junkyard with 133xxx miles on it and it works fine perfect now. Mine needless to say was over 207xxx miles.
Wanted to let anyone one who is having the same problem so they dont have to go through the trouble I did. My XJ is a 91 Laredo. Dont know if the newer Cherokees have this kind of problem.

 

You need a new VSS.

 

My speed sensor is fine. I spent alot of time checking all the sensors and conections. Theres a spring in the needle of the speedo that will wear out in time and is hell to replace from what I hear.