I thought '91 was the first year they didn't use the disconnect with the 231. In any event, my experience has been that with a vacuum leak the light won't go on because the disconnect won't engage. If the light stays on, I don't know if that's an indication that the disconnect is engaged and won't disengage due to lack of vacuum, or if it indicates a different problem.
If there's no vacuum disconnect on the axle, look for a switch to control the light right on the transfer case.