I am having some issues with my 1996 F 150 4 x 4. For some reason the four wheel drive will only work in HIGH gear. I tried to shift it into LOW but it won't even budge. Any ideas why it will not go into LOW gear? It's about to snow like hell in Telluride, Colorado and I really need to make sure my prized 1996 F-150 is running top notch! THanks