AWD is easier to drive on ice and snow than 4wd, though you can convert it to selectable 4wd if you really want. There is a thread somewhere that tells you how to change it to a manual transfer case from an F150. The AWD transfer cases are EXTREMELY reliable and shouldn't cause you any problems as long as they are maintained properly.
There could be a lot of things causing the rattle This is, after all, a 16 year old SUV you are looking at. Things get loose.
As with any vehicle, you should have a mechanic inspect it, or at the very least, someone who is experienced with buying used cars and is mechanically inclined. The 302 is a very reliable engine, but it is subject to wear just like any other. Standard inspection procedures. There aren't many things that stand out on these as a problem area when compared to average cars. Exhaust manifolds are prone to cracking I suppose. Check operation of tailgate, power windows, doors (inside and outside) and rear wiper. Don't be surprised if the rear wiper doesn't work... That seems to be one fairly common problem. Anything else is standard maintenance issues that you should check on any vehicle.
Good luck with your purchase, and if you decide to go with the Explorer, remember, we love pics on here.