All first and second generation explorers have 8.8 rear axles with an offset center section. The earlier, boxy explorers 91-94 have narrower axles (57ish) and drum brakes. The later 95-01 trucks have a wider axle (look at jeeps for the width, they love to convert to them) and disc brakes. Either style usually has gears ranging from 3.27 to 4.10. The door tag will have a section for 'AX' and a code underneath I believe all ford from the 80s on up use the same codes to determine the gear ratio.
All Explorers up to 2001 had solid axles. 91-94 had drum rears. 95-01 had discs. These are also all the same width. The disc brake rears "may" be a 1/4" to 1/2" wider due to thicker material of the rotor center. Other than that, they're all the same width. The axles are interchageable through all these years. Like others have already stated, check the metal tag on the center section to find the ratio and look for the "L" in the midlle of it to confirm it has a traction lok. They came in all ratios from 3.08 to 4.10, but 99% of the ones I find around here have 3.73s and traction lok.