The first thing to do would be to check the fluid level. The next step would be to disconnect the shift rod from the tc arm and see if you can manually shift thru the modes by hand.
Big Bro 5.5"/33's and Lil Sis 3.5/30's
https://photobucket.com/share/97196184-83a0-4a0b-9ac5-986f630a70d6
https://photobucket.com/share/cab4ce27-ed34-4301-b4bb-ab2bb31e3b1b