1989 F-350 -- 4x4 indicator light stays on

I recently purchased a 1989 Ford F-350 Super Duty Crew Cab w/ 4wd.
When I have it in 2wd, the 4x4 indicator light on the dash stays on.
I verified that the hubs were free, the shifter was all the way
forward, and that the front driveshaft spins freely. I even gave the
relay for the indicator a good tap, but nothing changed.

Any ideas?

Thanks.