I live in the snow belt, and agree that 50 years ago you had to make some provisions to protect the body from salt and rain. At that time (and perhaps today) there were many outfits that simply sprayed on undercoat that ended up holding water and salt close to the metal and made things worse. Always use a wax based (self healing flexible) product if you are going to do this. That being said, I have never had a body rust problem on a chrysler product since ... oh.. 1980 when better metals and plating certain parts became widespread (CJs still rusted). I stopped undercoating about that time. My 2000 JGC has 200000 miles over 13 years mostly in snowy salty weather and is without a hint of body rust. Both my 1980 and 1990 Voyagers went 15 years and the body looked new when we traded it.
The best thing in snow or ocean side country is to rinse off the harmful road spray often and park in a unheated garage. The biggest corrosion problem these new JGC have seems to be the "poor" quality chrome pieces that the spray on crap Connecticut likes to use on their roads will discolor and stain within hours.
Today undercoat does more harm than good for a vehicle used typically on the street.
Spray on undercoating being bad for rust protection defies the common sense of most, but facts are facts. It is worse than nothing statistically. Some posers like the looks undercoating gives to the car, and it can be useful for sound deadening, but the WK2 is pretty well sound insulated.
2011 Grand Cherokee Overland V8, 2009 Liberty Rocky Mt V6, 2000 Grand Cherokee Laredo I6, 1979 CJ7 I6 Quadratrac