See if the dealer/seller of the 2016 rig will include in the deal, changing the 4200/350 pound hitch to a 5000/500 pound rated one. Many 2016 MB chassis rigs have a 5000 pound hitch. They should be available. Tell the dealer what you learned about the towing problem with your Cherokee. If he includes the 5k hitch in the deal, then you are good to go ahead with the deal again. The cost of a new 5000 pound hitch to a dealer will be around $100, and his own labor cost even less so I think he would agree to that.
If your GCWR is overweight thereafter, then adjust your load accordingly by carrying less on-board water and such.