Assuming that even when I lock in 4 wheel drive, I am only really pulling with 3 wheels aren't I?
Well actually only 2.
You can get an aftermarket LS units form many different sources. It should be around 400-600 bucks installed. If you went with a Ford unit, you may feel better about the warranty part. But you can always ask your local service mananger about using aftermarket parts and how he (as your warranty provider) feels about it.
I should say, however, that living in MA and all the snow I get, I've never had a traction problem and I do not have an LS unit or A/T tires for that matter.
The sheer weight of the Excursion is it's ally in inclement weather.