Could be rear wheel speed sensors, cheap fix but it didn't fix mine. Or it could be what I have going on...
Took it to the dealership and they replaced the outer half of the ABS control module, $780.00. Didn't fix it. Came back and said they needed to replace the half of the module that is connected to the truck, left side wheel well in the engine bay and hub sensor for an additional $2300.00. When they said that I left to get a 2nd opinion but haven't yet.
Also since then a couple times I've had both speedo's stop registering any speed. I read that could also be related to the ABS control module and moisture. After talking to and showing the quote from the dealership to a mechanic friend but not having him look at it or diagnose anything he said the hourly rate the dealership was charging was 3-4x higher than average and the price for the ABS unit was quite a bit higher than Chrysler's listed price. I've just chosen not to use that dealership ever again but I am afraid they may have been right on what part is causing the problem.
I wish you luck.