I troubleshoot things for a living. So I know milking all the possible symptoms means something to tech people who know how systems work.

2003 Blazer S10 4 door. 4WD front gearbox was replaced at around 50000 miles with a used I think older unit off a wreck. The original gearbox ran dry of grease/oil somehow and the gears literally crushed themselves up like an ice machine.


Forward 3 years or so, it has never given a blip of problem. This last weekends weather went below 0 degrees F with 10 inches of snow to drive through. No problems it worked fine through most the heavy stuff. Then while driving I did a 4H to 2H button push on the fly, all the light went out but it was in 2 High.

When in park or driving pushing 4H has no effect, it stays in 2H with light lit.
If stopped and in Park button presses do nothing. If in neutral and stopped pushing 4H makes 4H and 4L bulbs blink a few time than 2H lights and stays on. If I select 4L I hear the clunk noise of the shif, it goes to low gear range and I am pretty sure it is in 4WD, but both 4H and 4L lights are on though 4H seems a bit dimmer. If I choose 4H from this point again both 4H and 4L lights blink and it goes to 2H with light. Cycle repeats as described above.

This is an aux vehicle and it's still below freezing so will not be doing any checking till it gets warmer.

 

As usual I say remove the 4wd switch assembly, separate the two halves carefully and then reflow all the solder connections, especially the connector pins. I know this fixed the issues my Blaze was having. Try this and see if the buttons work correctly.