Did you find your issue I have a 2004 Jeep Grand Cherokee Laredo 4.0 4x4 I have the same issue with my wife's Jeep Runs great just speedometer bounces all other gauges work perfectly already changed speed sensor in transmission tail shaft housing any help would be greatly appreciated God Bless You

 

Sorry I'm late, first thing I would look is the ground wire for the dashboard. I just received the problem again recently but this time when I put in reverse the speedometer goes nuts. I did clean it up but I'll still bounce so I would try cleaning the ends of the ground wire, if that don't work then I suggest replacing it.

 

Go to Jeepcables.com and order the big 7 Jeep cable upgrade just put in your vehicle information and order put them on and it may fix some weird issues u didn't know you could All your connections will be good and More solid than the ones they put in at the factory or dealership.... jeep cables.com also has a video to help you with the installation it's easy and I promise you will thank those Veterans that makes them
thank You God Bless You

 

You have a short in the ground. It is typical use higher temp wire and splice in replacement usually right about the transfer case/transmission area

 

Im currently dealing with a similar issue but mine also missfire as well all my grounds look good but for the life of me I can’t think of wear elsewhere too look for the issue it developed one day randomly