actually you have located your problem perhaps. Again observations are very important and need to be "accurate". Basically, the "thing" should not complete getting into 4w drive "auto" (light lit) until the front and rear axles come up to speed IF you have thrown the switch while driving. Your observation is somewhat "unclear" as to whether you have turn the switch while stopped then get the 4w hi light then begin moving and the light goes out..... usually this will result in flashing which usually indicates that the front axle and back axle are not with "speed tolerances" and the system "kicks" out of control track to prevent damage. Of course, on the "low side" of things (ie. stopped and neutral), the system should "lock" (vacuum operation, etc) and go from there (preventing speeds from reaching a certain range). I suspect you have a clutch issue. You can check to see if your "brown wire" has power... that's what activates the clutch.... and then go from there, potentially, doing the "brown wire mod" (search) if there is no power.
