Wear on the INSIDE edge could also be excessive toe OUT, as well as too much negative camber. The larger wheel and tire combination would only further exaggerate/aggravate this condition.
Have the front end thoroughly checked (ball joints/tie rod ends/etc...) Also have them carefully check the upper control arm bolts (the alignment eccentric setup) I've seen them loosen up. It looks fine while the truck is sitting there, but as the suspension cycles, they allow the upper control arm to flop around like a wet noodle, and change the alignment.
Once everything is checked out, and replaced if needed, get a good alignment done. If there is nothing wrong with your truck (frame bent,etc..), you haven't lowered TOO far, and the shop knows what they are doing; then it's is very possible to get it aligned properly. Make sure when you take it in, it's loaded the way it most commonly is driven, with the fuel tank about half full. Also, I like to ask if I can sit in the drivers seat while they do it, or at least place an equivalent amount of dead weight in the drivers seat (cement bags/dumb bells/etc...)
Also, when they balance the tires, have them check carefully for both an out-of-round condition, as well as lateral runout of both the wheel and tire. It may require dismounting, and spinning the wheel by itself on the balancer. It is possible for a wheel/tire assembly to appear balance, but have a hop (out of round) or vibration (run out) due to these conditions. Also, with tires that large, it may be a good idea to find a high performance shop that knows how to do "match balancing". It will cost more, as they have to mount/dismount/re-balance each wheel at least twice, but it helps to minimize the amount of wheel weight needed to balance each assembly. This in turn can reduce the occurrence of high speed vibrations.
The bottom line though, is that those 22" rims are simply not appropriate for the suspension and design characteristics of the 2nd gen Explorer. Yes, people use them, but this thing is a truck, not a Cadillac. It already rides a bit harsh. Adding a lowered suspension, and reduced travel, and then putting rubber band tires, and large diameter rims only makes it worse. Tires have a certain spring rate to them. This will vary, based on a number of factors (tire diameter, rim diameter, rim width, sidewall height, number of ply, tire material, tire construction, air pressure, tire and air temps,etc....) The oem factors this spring rate into the design of the vehicle, based on the oe tire type/size. IE;, they can use a bit stiffer/harsher chassis setup, and then tune the ride with tire. Sure there is a bit of room to play. You can go a little bigger, if you're willing to trade ride quality for handling, up to a point. 22"'s on an Explorer goes beyond that point though, and will lead to problems. Lowering it already reduces suspension travel. Rims that large reduce it even more. When the suspension bottoms out, parts bend and break.
