If you are following the procedure on the driver visor for putting the truck in 4X4 then your front hubs probably need to be disassembled, cleaned, lightly greased and reassembled. It is also possible that the transfer case shift motor is not functioning. You can verify that by putting it in 4X4, get under the truck (engine off and in park), try to turn the front shaft by hand. If it will not turn the transfer case is in 4X4 and the problem is the front hubs.