2003 f150 4x4 want work

it has the gear shifter and when i go to put it in 4 high or 4 lo the light comes on but it dont kick in.thanks

check to see if the vacuem servo is working on the front axle they tend to rust up its located on the pass side on the back side by drive shaft

tried the 4x4 today and its working fine.only thing i did was check all the fuses.lets just hope it stays that way.thanks