My '94 SE 4.0 is rated @5000 lb towing. I often tow older FSJ's using a tow dolly (over 5000 total, no brakes). Mileage runs about 9-11 mpg interstate @ 70 mph. This setup works OK, but it takes everything it has on some hills, and still loses speed. If you are buying specifically for towing, a 5.2 V-8 will doubtless perform better. For really serious towing, tho- a larger vechicle, such as a Chevy 2500HD with a Duramax diesel might be the answer.
Chuck R.