2000 Blazer, won't shift into 4Hi, shifts into 4lo in neutral at low speed, but the light on 4hi doesn't even go on, no click, clack, or oomph when I press dah button. a friend suggested a fuse problem, but the fuse is okay.

 

3-button switch or 4-button (w/ auto) switch?

 

3-button. I don't think it's the in-dash switch, maybe control module, hopefully not a transfer case issue.

 

The # of buttons tells me what transfer case you have. You have the 233 transfer case which has been fairly well documented.

Have a look at the links below. Do not let the year difference worry you concerning the thread by error_401. Much is the same.
OBD2 Non-Automatic Transfer Case Diagnostics by lastcoupe & swartlkk
NV233 transfer case / encoding motor switch by error_401

And test the switch just to be sure that it is not something simple. The 3-button switch is just a continuity switch. One wire in and one wire out per switch. The diagrams are in the NV233 post above.