The 4.0 SOHC engine has very good compression, over 9.5:1 I believe. That will work very well with
high octane fuel, I would expect to see almost equal fuel mileage with a 91 or 93 tune, versus an 87 tune. Meaning do what you are planning, but expect to be very satisfied with running the best gas full time. Al has been successful with leaning out his 87 tune a lot, but his priority is the fuel...