Nevermind, figured it out. Mods please delete.

 

Minor correction: Both directions of the actuator are vacuum driven, that's why there are vacuum connections on both sides of the diaphragm.

The switch is operated by the actuator so if the light comes on when going into 4wd that part is working correctly. More likely a problem with the shift fork or collar, the cover is easy to remove.

 

Ok thanks.

I pulled it off and the collar and fork look to function fine. The axle does connect and disconnect depending on where the t-case level is...but it's backwards. It locks the axle together when in 2hi, and disconnects in 4hi/4low. It isn't as simple as the lines being installed backwards because the lines were not touched while the failure occurred. It was working fine beforehand I believe.

 

Ok here's a weird twist. I pulled the vac lines and swapped them. I now have functional 4wd again, but the indicator light is backwards. Showing 4wd while in 2hi and no light in 4hi.

The only thing I can think of is maybe I re-installed the whole actuator assembly upside down when I did the axle seal a few months ago. My vacuum canister is on the driver's side, and the photos I see all seem to show it on the passenger side, but I thought maybe they changed it mid-cycle. That still doesn't explain how I had working 4wd on the way out to help this guy though...

 

When does the 4WD light come on? Look at this it may help.

 

(Eric O. voice): There's your problem lady !