OK, You have 2 options...
OPTION 1: You can swap over a double cardan driveshaft from a newer grand cherokee. This would require you to also replace the yokes at the differential and at the transfer case. Someone will have to chime in about the difference in length of the driveshaft, if any, between the I6 and the V8. This would be by far you cheapest option. When replacing the pinion yoke you have to be extra careful because the nut holding it on put a preload on the pinion bearing and when you replace the nut it has to be in the same place to put the proper preload on.
OPTION 2: You can call up Carolina Driveline or Tom Woods to custom make you a drive shaft. They can pretty much set you up if you tell them you situation. I believe this cost around $300-$400