I'm having the same problem ! To answer your question though, even if you locked the hubs manually, you won't have power unless the transfer case is engaging, which it probably isn't if the light's not coming on. I'm assuming you have ESOF and not a manual shift TC. A vacuum problem will keep your hubs from enaging (hence the manual option) but the TC is shifted by an electric motor, not vacuum. What kind of truck do you have?