During cold winter weather, one of the rear wheels on my JD 2630 will lock up as if I stepped on the brakes. It happens to one side or the other, but not both at the same time. It only happens during cold northern Minnesota weather - 20"s or below. The tractor is parked inside an unheated, but insulated garage. When I back it out, it"s fine, but after I drive for five minutes or so (to move snow, etc) one of the rear wheels will "lock up". After letting it sit for a while, I"m able to nurse it back to the garage. If I run the garage heater and warm it up before using it, it does not happen. When it does lock up, I cannot move the brake pedal, it seems to have pressure pushing it into the "off" position. Any clues?