A funny review I found here:
Grand Cherokee Overland Diesel Review | The Truth About Cars
The author sure hates diesels!
Read the review... ... I thought the 6. 7 Cummin's was a quiet diesel.
You should be ok with this engine as I believe it is the same block that they used in the Jeep Liberty CRD. I had a Liberty CRD for approx 2 years and was getting 22 to 26 mpg with the 2. 8 liter. I have read that you can expect around 18 to 20 mpg with the 3. 0 liter. good luck with your purchase.