I was a chrysler tech a few years ago and our shop had 2 grand cherokees with 4.7 's and both had a burnt exhaust valve on # 7 cyl. The first on came in with only 18 k on the odometer and the second one came in with about 23 k. on it with the same cyl. I did both of those repairs. Those 2 jeeps came in about 3 weeks apart. I couldn't believe at low miles and the same cyls. on two different vehicles. Sent both heads out to a local machine shop and the shop stated that the exhaust guides had not been machine correctly and the valve wasn't seating properly. If I remember they were both 2000 models. This was about 2001 before I had left the dealership. At the time I had a friend that worked at another dealership and had ask him if they had seen any issues with 4.7's and he said they haven't. This was several years ago. Not sure if this is your problem, but thought I would let you know..