Stay away from underdrive pulley kits. They lower your water pump and alternator pulley speed, which will lead to inproper cooling, especially with mechanical fans - will spin slower. Your lights are dimming because you are in fact turning your alternator slower. All your electrical system has a preset draw, or load, which will not change, it will always require x number of amps. Now that you spin your alternator slower, you have to have your engine rpm higher to get the same output. Under drive pulleys are for drag cars, because they dont care about stuff like this.
Actually, the biggest irony on earth is underdrive pulleys, because most people go to high volume water pumps and HO alternators because their cars overheat and dont charge properly, which in the end neutralises all the effects of the underdrive pulley. I trust you are not building this thing for 1/4 mile times?
I have 33 x12.5 on my stock engine with k&n filter, I dont have problems off road. A lower 4low would be nice, I might get 5.82 gears later, but it only is a problem when the mud is really soft, the kind that grabs your tires and holds it.