Is selling this Jeep and ponying up the extra cash for a 4.0L TJ possible given financial considerations? A year or so ago, they were going for a solid $2k more than similarly equipped 2.5L Jeep.
How much time and money do you want to invest in this, long term? (sort of a rhetorical question.)
If selling this Jeep to spend more money on another isn't an option and you're looking for minimal investment to get out into some harder terrain, I think luvmyxj's advice is spot on. You will want gears too. ZJ steering is cheap and easy, and prevents an annoying trail fix. Stock gas tank skid is useless, get a real one.
If you have a D35 rear (which I'm pretty sure 2.5Ls universally came with)... that sucks. Mike Wolf, Gari, and others make them work, but I personally won't advise spending a dime on that axle. Ford 8.8 is a great swap, but relatively expensive on a Jeep with a 4 link rear.
If you can swing the extra cash to upgrade to a 4.0L TJ, that is a decision you will unlikely regret. It will also be a much better platform for a long term and more involved "project build."