Google the part number of the radio and research to see if it a low or high speed bus radio, I've seen more than one place getting it wrong
Yes, it sounds like a bus mis-match.
The earlier Grand Cherokee ones share the mounting pattern with the RAM so they'll bolt in, but they're low speed bus (RAM is high). The later GC ones are high speed, but they don't have the right mounting points.
I also believe the RHR replaced the RER in the GC when they changed the bus speed and mounts, so I don't think there are any RERs from a GC that will work, regardless of ****.