Which model are you talking about. I have 2004 grand cherokee which has petrol engine 4500 cc. The newer 2006 and upcoming 2007 have 4700 cc petrol engines.
As a rule of thumb any car can be converted to deisel but it will not be bolt on job. Engine bay will require a lot of cutting / welding to make new engine fondations. Many engines can be used mainly from Toyota, Nissan, Mitsubishi and Isuzu. Isuzu 2800 cc turbo engine presently used by Adam motors Boltro is a good powerful deisel which will be suitable for Cherokee.
My brother in Lahore has 1972 Cherokee wrangler and he has converted it to deisel with Toyota 1C. The car is running good as its other parts are all genuine and its a comfortable drive. However more specs of your car and pics will help in giving you some more suggestions.