do you mean what's the most important modification you can make for off-roading, or the most important spare parts to bring with? If we're talking about modifications, first most important is manual hubs. Second most important is better tires. Third most important (and some people disagree with me here) is a winch. Basically anything else you buy for your Explorer (lockers, new gears, suspension lift, off-road lights, etc.) are merely to prevent you from getting stuck, none of them will actually get you un-stuck. Once you have the winch then fill in the other things later. If you only plan on doing easy to moderate wheeling, you have no reason for a winch and should instead spend your money on lift's and such

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By the way, some of the best spare parts to bring along are oil, transmission fluid, and grease. I'll tell you what I usually bring along with me in my box, is all my wrenches, sockets, drill bits, and hand tools (hammers, etc.) for tools, I also bring along a spare tube of grease, and extra set of brake pads, plenty of zip ties and duct tape, spare u-joints for both my axle as well as my driveshaft (even though I'd have a very hard time replacing them on the trail), a tire repair kit (these proved useful more then once at the CCR2000!) a can of fix-a-flat (even though it won't do much good on bigger tires, but it might help someone else out), a few spare spark plugs, jumper cables, an extra positive battery cable, and other various bolts, screws, cotter pins, etc.