Re: Stuck in the snow
Get a detroit tru-trac or full locker. They're the best on the market and you might find it at an OK price, when compared to other locking diffs. The Eaton fuller Detroit unit is fully automatic. Once it detects the slightest slip, it locks. I had one installed in my old GMC, worked fantastic even with nearly bald tires in snow. I was tires of the open differnetial (one wheel peel)
An LSD, or limited slip rotates power back and forth to the wheel with the most grip. It shouldn't keep spinning just one wheel, or the unit is most likely malfunctioned and should be replaced anyway. LSDs do have a lifespan as the clutch packs can wear out. if you're easy on it, it'll last the life of the vehicle, but burn outs, donuts, off roading takes it toll.
Another option, although it's pricey and more for the hardest core of offroaders, is an ARB air locker. I'm sure they have one for the Excursion, but they're pricey units to buy, and get installed.
You can lock the front hubs, which locks the wheels to the axle only. If the transfer case isn't in 4HI or 4LO then the front axle isn't connected to the drivetrain, and thus, won't provide traction. Once you lock the hubs and get into 4wd, will the axle give you power.
If the transfer case won't go into 4HI or 4LO, and you have a selector knob on the dash, then it's most likely a problem with the TCCM, or transfer case control module. It's a common on older vehicles for this part to fail, irregardless of manufacturer. If you go through the dealer, it'll be rather expensive, best bet is to check a local salvage yard and get a unit from a vehicle that had low miles. if you have a manual transfer case lever on the floor, and it won't engage 4WD at all, or has a lot of resistance, then he selector forks that slide inside the transfer case to engage a different drivemode are most likely broken from use/abuse. The best thing here without being able to check the vehicle out myself is to take it to a shop you trust for a proper diagnosis.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!