2001 Chevy Blazer LT 188,000 miles. Here's the rub. I checked my 4WD and low and behold the 4 HI light continually flashes and won't engage. I put it in 4 Lo and the light flashes but you can definitely feel it kick in. I replaced the vacuum switch on the engine fire wall and no change. I am not sure where to look next. Any/all advice deeply appreciated. Thanks...

 

Check the connections on the TCCM in the passenger side kick panel.....Is the service 4 wheel drive light on ?

 

It came on once. I was done for the day and shut it off. Now it hasn't come back on. Is the TCCM directly behind the kick panel?

 

Yep, directly behind the pass kick panel. Clean any corrosion and put back together with some die-electric grease.

 

You could spend a couple of hours searching and reading the many postings on things to try. But in addition to checking the TCCM connections, remove the 4wd switch from dash, carefully separate the two halves of the switch and re-flow all the solder connections on the circuit board within, especially the connector.
Easy and pretty much $0 cost.