Just thought I'd chime in here with a slightly different perspective.
First, when you say "Winter Beater", what exactly do you mean by that? Do you just want something that will handle in the snow? Will you ever take it off-road? or do you just want something that, if you spin and crash in the snow, you don't really care? There are a lot of ranges.
On the IFS and IRS, I can say they do all have their place. There are several problems you would run into. The first would be finding the room to fit all the suspension components, but it can be done. The second is that the newer Explorer's are wider (Both Suspension wise and Chassis wise) than your '91. This means that you would most likely have to either widen the chassis at the points where you mount the IFS and IRS Differentials. Also, to take advantage of having front and rear suspension in the snow, you would want to swap out your Transfer Case for an automatic one, something like the control trac. This would allow you to put it in "AWD" mode and, if any of the wheels slip, it will engage the front axles so you don't spin and die.
There's a lot more than just that involved in a swap, but that's kind of a start. Now, in terms of why you want to go to IFS and IRS. If you're sole purpose is to drive to and from work safely, I would say that the IFS and IRS would be perfectly fine. Its main advantage is that it does a great job of keeping all 4 wheels on the ground. The problem is, due to the shorter A-Arms, the amount of travel you get is very limiting if you ever take it off-road beyond a small mud puddle or some dinky rolling hills.
Also, comparing Humvee Suspension to Explorer's Suspension is like comparing an R6 to a Tricycle, they are totally different classes. Humvee's A-Arms are about a foot longer then the explorers, plus they have the Gear Reducing hubs that raise the wheels an extra 2 inches more off the ground, getting the frame that much more clearance. They also have the entire driveline hidden behind blast shielding so that no road grime can touch it. Plus, if you've ever sat in a Humvee (I have 3 friends that own them, and I do mean Military Humvee's, not Hummers) you will know that they don't really fit on the roads. On a wide freeway they have about 2" of clearance on either side before hitting the markers, on city streets, your strattling lanes. But, its because of this wideness that makes them such great off-road vehicles.
I would say your best bet for what you want would be to swap out the front TTB Suspension for a Solid Axle Dana 44, then put the whole suspension on air bags. Air bags give a soft ride and would allow you to run it lower on freeways (lowering your center of gravity) and then raising it up some if you want to go off-roading, but you will probably need to do 4-link suspension all around and move the bags half way up the links to get enough variation in height to get any real lift/lowering ability.
It is all possible, its all a matter of how much time and money you really want to spend. But, if you just want a reliable machine and something to work on, I would say fix what you have now. Replace all the rubber components, new shocks, maybe new springs, then dig a little into the engine. You can have a lot of fun working on the X without tearing the whole thing apart. Custom Bumpers, Sliders, roof racks, exo-cages. You can do almost unlimited things to improve its abilities without ever touching the suspension. And the more you know about your vehicle before you tear it down to its skin, the better your chances are of getting it back together.
Man... I've gotta work on my long threads, sorry guys
Thats cool...if thats what you want.
). If you want to "refresh" the suspension, you can get some new bushings, coil springs, and leaf springs for it and it's good as new. 
