The light on the dash is controlled by the vacuum cad on the front axle housing (passenger side) un bolt it and have someone shift to 4 wheel drive and see if the fork slides over quickly. If it is slow or weak then your cad may be shot. Also could be a vacuum line between the transmission and the cad or between the transmission and the vacuum canister (drivers side under brake booster). Third possibility is that the little valve on the transmission that has 3 vacuum lines going into it is at fault and isn't allowing vacuum through. Hope this helps!