Let's say the truck has steel front bumper with winch, double swingout rear bumper and sliders. A mild lift and good tires. Otherwise, its completely stock.
Your truck is already built. Just go on a trip to learn your priorities. A common mistake is to focus on the truck. I'd recommend you focus on the trip. If you're really asking how to prioritize spending, here's a few thoughts having done just this for 15 years.
-A good spare tire that matches your existing tires $200.
-Delorme or Benchmark mapbooks of all the states within a day's drive of your home location $75
-A basic set of tools-$150 and an afternoon of careful shopping at Harbor Freight
-Refrigerator $500-$1000-really this is the best camping bit ever.
-Get some camp chairs- $100 for 2 folding GSI chairs
-You didn't mention a skid plate. The 80's stock skid is a joke. $250+
-Kitchen gear-this can be pretty cheap or even brought from home and thus free. A skillet, some pots, a Craig's list stove. $100
-A way to carry extra gas-10-15 gallons is about right. Even cans work fine on the rear rack $100-$150
-Recovery gear-at least 3 rated shackles, 30 foot recovery strap, 10 foot tree strap $150 or so, leather gloves.
-Dual battery system. Blue Sea SI-ACR, some 2 ga cable and you're done. $150 (avoid pre-packaged systems)
-#2 Battery-deep cycle group 27 - $100
-Tray for #2 battery $100
-Wire a fuse block inside your truck with accessory circuits, fed by the #2 battery $100
-Water storage-recommend 10 gallons on board. $30 for 2 reliance blue cubes plus:
Amazon.com : Aquamira Water Treatment Drops (1oz.) : Camping Water Purifiers : Sports & Outdoors
-I agree with the idea of being able to sleep inside. $Free - $250 if you build a platform. You don't need a tailgate tent.
-If you go desert camping, an awning is a splurge but very nice to have at times. $50(tarp)-$500(attached premium awning)
-Some storage-Drawers are nice and cool, but they take up room and are themselves heavy. Use plastic boxes for now $50
-Radio-Kenwood 281 or Yaesu 2900. $250 with antenna ($150 radio, $100 antenna)
-Costco pack of ratchet straps to tie things down - $50
-GPS-Use your cellphone to get started. Very low priority.
-A Hi-lift is only very occasionally useful. Low priority, get it later, or not, but if you get one, get it with the wheel lift accessory and base
Things you definitely don't need:
-Propane campfire-this is for the wankers on Expedition Portal
-Anything made by Snow Peak - it's like buying a Rolex when you need a Timex
-Roof rack at least at first, but if you get one, it gives you a place to attach the awning-see above!
-Off road lights
But if I were you, I'd get the spare tire, the fridge and the camp chairs and go somewhere interesting. Spend all the left over $$ on fuel for the beast. I'd also advise you invest time in making some friends to go offroading with. A working non-stuck truck and a buddy is about the most useful accessory you could have.
Your needs and wants will continuously evolve. That's normal. So adapt, improve, go forward. You may find on a trip you need something you don't have. Buy it when you get home and you'll have it the next time. The $3000 you spend now is not the last $$ you'll spend. Just remember you can go on a trip tomorrow for the cost of fuel. So keep things in perspective.