My truck wont engage into 4wd even though the indicator light says it is in.

What is your question?

When I push the button for 4wd, the indicator light comes on and it sounds (from the front axel) like it is engaging, however it doesn't. Is there a part under the truck that can become stripped or needs to be replaced that engages the axel? The service 4wd light is not on, however there is a check engine light that indicates a vacuum leak.