1998 Blazer no 4hi

This is what Happens..So when I start my blazer the 4HI 2 and 4Lo light all come on and shut off.If I go too Neutral I can shift it into 4Lo and the light will come on then when I push 4hi nothing but I can go back into 2 wheel... When in two wheel I got no lights on the three way switch.. Any Ideas

Make sure the connections at the TCCM in the passenger side kick panel are clean and tight, (no corrosion). Make sure the connections at the encoder motor on the transfer case are clean and tight, (no corrosion). Make sure the actuator under the battery has good vacuum to it when 4WD is selected when idling in park. Make sure vacuum at the actuator is immediately released when shifting back to 2WD. Make sure the actuator cable moves freely, (about 3/4") when shifting from 2WD to 4WD, and back to 2WD.