98 Lincoln Navigator 4X4 low wont disengage

My 98 Lincoln Navigator switched into 4x4 high on its own and could not get it to diengage. The 4x4 light works fine, but when I try to switch it bake to 2wd on the control trac system on the dash, nothing happens. I ran it like this for four days and then it switched into 4x4 low on its own, I had to drive it really slow on the street back home and have not moved it since. The 4x4 low wont disengage. Can anyone help so I wont have to take it to the dealer.