18, not a lot of $$$ and need to spend smart.
You just made your first investmentment in modifications and it didn't cost you a dime by starting down the correct trail by asking for advice. North on 101 just outside of Ukiah is Cow Mountain and a little further north and east is Lake Pillsbury. There are plenty of trails there that can be fun and challenging for a stock vehicle. Bryan, Jon and others are correct. Learn your vehicle and your own limitations by getting some seat time. XJensen mentions custom built rigs that were having difficulties and Rightseatsis and I have experienced this also.
That said here is my advice. You spent good money on your baseline rig so you'll want to keep it and protect it.
Keep it:
1. Wheel with a friend, don't go out alone until you are well prepared.
2. Recovery points. Get some tow hooks with brackets, snatch strap, D ring shackles. You can use a rear trailer hitch as a recovery point
IF it is secured by nutstrips.
3. Look up the minimum requirements needed for a

trail run, recoveryu points, fire extinguisher, battery hold downs. They are not burdensome or tough to meet but will allow you to keep your Jeep.
Protect it:
1. You have a front skid & rear skid. Next get rock rails or rock sliders to protect your investment and rocker panels.
2. Next get a transfer case skid. With your previously purchased and installed recovery point the friend you are wheeling with can drag your Jeep over obstacles.
Since you have a limited budget, these purchase may take a season or two to complete and by then you'll have seat time to start thinking about the other modifications that were suggested by the posters on this forum.
Plan on attending a group trail ride somewhere. Sierrafest is coming up and if you really save money and eat Top Ramein the 30th Annivesary event in October would give you a wealth of experience in a short time.
Have fun.