I know it's been a while since I've been around, and apparently everyone with this problem 2 years ago has either left, forgotten or is just not around anymore...
This is a common problem on track/kicks that are offroaded. Changing the fluid wont solve the problem and parking it with the sticks in special places is merely a way to deal with it, but not fix the original issue. The transmissions are top loaders with the shifter coming right out of the top of the tranny. The linkages are in their own chamber, not the same chamber that the oil runs in. What happens is water gets in around the stick's base and freezes, making it hard to move the linkage. The best way to fix it is to park the truck in a heated garage overnight so that it thaws all the ice, then pull the console out, invert the in-cab shift boot(stretch it up out of the way) and do the same to the floor shift boot as well, pull it up and out of the way. Now you can reach through the hole in the floor, find the base of the shifter where it goes into the tranny, and push it down giving it a little turn counter clockwise to release it and the shifter will come out in your hand. Use a large syringe, shop vac or some other means of suction along with a piece of tubing to stick down in the hole and suck out all of the water that you can(you'll be amazed how much there can be in there). Once you have gotten out all of the water that you can, blow a hair dryer or heat gun down into the hole if you'd like to try to get it even dryer, then blast away with WD40 trying your best to cover all of the linkages inside the hole. Once it's all dryed and sprayed, put everything back together (just like you took it apart, but follow the steps in the other direction) and you should no longer have the freezing problem until after the next deep water crossing! LOL!