My '01 (can't speak directly to your '08) has the fan/waterpump pulley driven by the back-side (smooth side ) of the belt.
After chasing around, the squeak I had on cooler mornings or after sitting a few days was the belt slipping on the water-pump pulley. A gator-back belt (Goodyear) together with breaking the sheen/shine off of the pulley did the trick.
I worried a bit that the water-pump was getting stiff and requiring additional force to turn, causing the slip condition -- I was able to suggestively compare a few Ranger (and other 4.0) water pumps and the pump on this '01 didn't seem to require any additional force to turn.
I'm not sure that there was a good conclusion -- I'm fairly certain that the noise will return as the belt and pulley wear again.
As a diagnostic step - ONLY after you've committed to changing the belt, try adding a bit of light lube to the smooth-side of the belt while the squeak is happening... if it fades, there's only one (at least in '01) item driven off of that side of the belt. Of course the suck is that you'll need to pull the belt and clean off the contamination that you just added before replacing the belt....