4wd Problems

I have a 92 s10 Blazer 4 doors with 4wd. My 4wd has worked forever until just recently. For some reason when i shift the truck from 2wd to 4wd the 4wd wont engage. Its not an electric shifter. Now it is easy to move it into 4wd but when i wait for the lights to appear and for the 4wd to engage it never happens. I also try and listen to hear if it engages but it doesnt. I have no clue where to start to look. Hopefully I wont need to replace the transfer case......

Any ideas..............?

it's possibly the vacume actuator located under the battery. the rubber blatter cracks and won't let the front axle engage.
do a search there is alot on this subject
i just replaced mine this winter.90 bucks

Thx I just read an article about how to replace that in the FAQ section