In this order:
1) Headers:
You will get the most noticable performance gain here in both HP & torque. Lay out the bucks and get good JBA headers ( ~$450 last time I looked)
2) Throttle Body:
Note: The throttle body won't add much at this time except for drivability. It will be more valuable after doing some of the head work. Now or later does not matter. Besides, you won't notice the TB for performance gains until you run at higher RPM (>3500) when you need more air. (BBK 66mm TB ~$190)
3) Head work:
TM98 heads, bowl port, mild runner porting, some exhaust port matching, and polishing. (I am not a real believer in polishing the combustion chamber unless you plan on doing it yourself. It's a lot of labor cost that really won't give this engine any added performance.)
New heads - TM98 ~$600 (pair)
Porting Labor - Mild port job ~$300 - $450
New gasket kit- $80 (must have, head rebuild gasket kit)
New head bolts- $35 (must have, DO NOT use old bolts)
Note:
A) There is an aftermarket valve train kit that is a bit better than the stock from Delta Camshaft OEM replacement with hardened push rod cup. Phone: 1800 562 5500
B) If you go for a new cam you will also need to replace the valve push rods, and springs. You should not need to replace the roller lifters.
4) MSD or Jacobs Off Road ignition system.
Better combustion (multi spark) at low RPM and on inclines where you need it wheelin (~$350).
Depending upon your priorities....
Before working on the engine you might consider some drive train / suspension modifications aimed at increasing your off-road capability. Body / suspension lift, larger tires, lower gears might be next before you tear into the engine.
Yes, there is always something else to consider.
Be content to make your mods one at a time as your budget allows.