is there a freeze plug on the back of motor

There are unfortunately, three plugs back there. Two are water and one plug is an oil plug of which you should never have an issue(it's lubricated with oil not water). The two water plugs are located, one in the center line of the engine block and the other is higher in the head. I am told and can now confirm that when the engine blocks came out of the their casting stage, they were not cleaned/purged of the casting sands. When I popped out my lower water plug, the amount of sand that came with it was astounding! Typical Chrysler lack of quality control. The sands just grind away at the softer plug material and eventually you get a leak. Installing Brass plugs should prevent this. Also, before installing new Brass plugs be sure to clean as much sand out of the area and clean the opening where the plug will seat. Add some Permatex to the plug outer ring to help seal that area. There are a total of 7 water plugs on the 4.0L I6; 5 on the left side and those two on the back. Luck to ya!....

There may be a freeze plug at the back of the head and even freeze plug inside the bell housing/behind flex plate on the block, my illustration was not good enough to verify you would have to pressurize the cooling system first to look for leaks and then call the dealer regarding the freeze plugs, if one is leaking all are probably corroded.