2000 chev blazer lt 4wd 4button dash switch service 4wd warning light is on when i try to put it in to 4wd button blinks it tries to engage but doesnt then 2wd button lights back up what is wrong?

SOURCE: my 2000 blazer won,t select 4wd hi/low or auto

there is a vacuum solonoid on the firewall in the middle with 2 vac lines and a 2 wire plug
thats ur problem
push the button and smack it with the handle of a screwdriver should work

SOURCE: My 98 chevy blazer won't shift into 4wd.

Most common problem is the vacuum diaphragm that is so inconviently mounted UNDER the battery tray.The cable that locks in the front diff is attached to it and vacuum pulls it in/out.

SOURCE: My 1999 chevy silverado 1500 four wheel drive

You probably have a bad encoder motor.It is located on the side of the transfer case on the drivers side.It is black.The other problem they had with these was the switch but you have already replaced this so your next thing is the encoder motor.Hope this helps.Good luck.

SOURCE: 4wd not engaging

These trucks are famous for the transfer case control module failing
and causing problems like you are experiencing. The module is
usually located in the passenger foot well kick panel. You should
be able to find it easy since that is what is making the clicking noise
when you try to go into 4x4. When you find it-unplug the 2 connectors
and looks for green corrosion on the terminals in the plug and also on
the module. They are famous for water intrusion thru the heater system. If you find corrosion, terminals in the connectors will need to
be replaced as well as a new module(they are expensive).

SOURCE: chevrolet truck 4wd switch blinks

If they are blinking I believe it means that there are fault codes present in the pcm. You need to get the computer scaned to see if there are an codes present. If you live near an Autozone or Checker autoparts they can scan it for free.