-Out of the ones listed I'd say the first one. It looks good (from the picure). AWD usually has worse fuel economy so if you could get a 4wd one it'd be a lot better, but AWD and 4WD are about the same besides AWD doesn't have a 4wd low setting.
-As for how much will a dealership drop, it all depends on what dealership and how much they are willing to. My 05 they were willing to lower the price quite a bit. But other dealerships wouldn't lower their price more than $200.
-The 05, has way too many miles for that year, and that differential would turn me off there. Obviously the person knew there was something wrong with that X.
-My 05 its rear diff is silent. That 05 just seems like there is a lot of problems, and it wouldn't be worth gambling on it.
-My 98 has been a great car. Has 170K miles, and runs strong. I have had the front suspension basically rebuilt...and a new head gasket put on it. But other than thats those second generations (95-01) are great. Mine is the 4.0L V6, but I've heard good about the 5.0L.
-And do search for a private owned explorer like said by sean99 they are usually cheaper. If you can try to find a one or two owner explorer.
Ask for a carfax on all the dealer cars you look at.
Those carfaxs can uncover problems that you wouldn't otherwise know about, and it will say how many people have owned the car.
Good luck with purchasing your vehicle.
Thats a good looking mountaineer. And at a good price.