Cheapest? new/reman 2.5. Best? Haha
Everybody is basically right, if you are looking for just a little more hp, use the money you would for a swap, sell the 2.5 jeep, and go buy a 4.0 Jeep.
I've been researching options for a while now, as my high mileage 2.5 and ax5 probably aren't long for...