4 wheel drive doesn't lock in
I got a 1990 GMC 4x4 that I bought new and have maintained well thru the years. I used the 4 wheel drive about two weeks ago and it worked fine but three days ago I tried to lock it in and it wouldn't lock in. It has the shifter in the floor which is where the light is located letting you know it is locked in. The light don't work and the front wheels don't pull. I changed the acuator thinking it might be weak but it didn't fix the problem, also checked the new acuator before installing it and it checked OK. Anybody had this problem before, I need help.