My truck runs fine until I turn on the fan, doesn't matter if it's heater or air conditioning, it acts like it's missing. I don't get any codes or have a CEL on. I've recently replaced plugs, wires, alternator, water pump, thermostat, heater core, u joints, fuel pump, and exhaust from exhaust manifolds back all within the past 4 months.
Anyone have any ideas?

 

Im guessing youve checked for amp drop? if not id do so.
I cant imagine any amount of excessive draw would make the truck act that badly but who knows.

again, just a guess.