The lack of power is probably due to the gearing. You might have stock 3.27/3.73 gears on that thing, and with larger, heavier tires, it's going to have less power and go slower. The only real fix is new gears for the larger tire size.
Other than that, the best place to start is general maintenance. Make sure the air filter isn't dirty, that the MAF sensor is clean, the throttle body is clean, the spark plugs are good, the wires are good, the fuel filter isn't old or partly clogged, the coolant and thermostat work good, etc.
Also good to make sure the fluids are good, that the front/rear diffs have gear oil that isn't old or turning to goo, that the ATF in the trans and transfer case is good.
It'll of course cost money to replace stuff that needs it, but generally you can do a lot of maintenance work for under $100 in fluids and parts.
After that, you can get a good bump in performance with a $100 KKM intake kit, and a $150 Dynomax cat-back exhaust.
You can cheap out and get a generic MAF adapter and cheap cone filter, and get a muffler with a 2.0" in/out and clamp that onto the stock pipes if you want to do it for not much at all, but the gains are smaller than with the better products. Depends how much you want to throw into it.
None of this will be a replacement for lower gears, but it might make it a little more bearable in the meantime, and if/when you do get new gears, it'll just be that much better overall.
