I've run up and down with tire psi. I found it more tolerable at 60 front & 65 rear. Keep in mind I have the 6.4 hemi which weighs a lot less than the Cummins
I am suspect of the lack of mounts on the transfer case. I'm thinking the shock of a bump at highway speed is pushing up on the transfer case via the driveshaft before the splines can compensate. This sets off the up/down shake or vibration. There's only some small mount behind the transmission oil pan and 2 motor mount. The engine/transmission are a counter balance to the transfer case. Not a good idea and worse with the lighter weight gas engine and Chrysler transmission. My theory anyway.
I am suspect of the lack of mounts on the transfer case. I'm thinking the shock of a bump at highway speed is pushing up on the transfer case via the driveshaft before the splines can compensate. This sets off the up/down shake or vibration. There's only some small mount behind the transmission oil pan and 2 motor mount. The engine/transmission are a counter balance to the transfer case. Not a good idea and worse with the lighter weight gas engine and Chrysler transmission. My theory anyway.