Last year I troubleshooted a 4 x 4 problem back to the dash four button switch itself, a common issue, after removing, cleaning, and applying deox to the module pins and connectors.
All worked last winter.
Recently I tried to kick in the 4 x 4 as it hasn't been used in a while.
No matter what setting I chose, the light would just blink in the new setting until I hit the 2WH switch and that would again go solid. My take on the lights flashing was an indicating of the unsucessful attempts of the system to complete the switching.
First idea I had was to check at the vacuum actuator on the center of the fire wall. I found no vacuum there so the fix was as simple as tracing the vacuum leak back in the hoses to the manifold. I then proceeded to replace all the vacuum lines.
There is a real tuff one to get at behind the ABS bracket into the fender area, but the fitting is stationary so it can be done with a little patience. Dry rotted hose will usually break off and leave a hose stub on the fittings and that one was tricky.
Most of the hoses were quite dry-rotted and cracked very easily, so I likely caused the problem earlier in the summer when changing plugs and wires.
Just a heads up that if your truck is as old as mine, those hoses should be changed out.

 

Yup the one by the brake booster into the fenderwell is a BIOTCH!
It goes to the cabin controls for the vents etc I believe. I hate how it goes into the fenderwell & then you're screwed for anything easy to trace & replace.