Do you run the fan constantly or off a thermostat? I bought one from Autozone for my YJ and it would only kick on when I was driving slow or sitting in traffic in SC. It did pretty well. It was also only cooling a 2.5L and the radiator was new. You might to up the price and buy a better quality one.
I'm using the fan assy out of a Pontiac Fiero. My buddy had it as a spare and it fits the radiator perfectly. I use a Deraile adjustable controller. It's worked great for a couple of years.