You are right, there are some clutch packs in the differential. They do wear out and may need replacing. At about 195,000 miles the all wheel drive system seems to bind up when making a slow tight turn, more so in reverse. On gravel, backing up slow, tight turn, the rear wheels want to skid like maybe the rear end is not slipping. I have serviced the front axles, changed the all wheel drive transfer case fluid and the rear end grease.
Nothing seems to improve the binding.
Try changing the fluid in the differential. Be sure the new fluid is the one recommended for your van. The flat rate to replace the ring and pinion gears on your explorer is 4.0 hours.