Anyone think they know what this could be?

Your description of the suymptoms seems to indicate a control problem more than a mechanical problem with the transfer case or front axle. Testing the transfer case actuator/encoder and control module would be the fastest route to a solution.

Need more info on year, 3 or 4 button switch, how you determined etc to help.

1st Check vac actuator under battery tray for tears, or leaks, and that the vac hose is connected. Replace if found

2nd Check vac hose leading from vac actuator to vac switch on transfer case for cracks and dry rot. Replace if found.

3rd Check remaining 2 hoses from vac switch on transfer case for dryrot or cracks Replace if found.

If all of the above are fine, disconnect your negative battery cable for 1/2 hour then reconnect it. If your 4x4 works after that you probably have a failing TCCM (Transfer case control module)

If that doesn't solve it, it's most likely the encoder motor

Thanks For The Info. I Will Pull The Battery Out And Check The Hoses First.

By The Way: 3 Button 2000 Zr2

I Found Out Buy Driving On The Beach. Most Times It 4hi Just Blinks And Goes Back To 2hi. 4lo Will Go In Most Of The Time. It Actually Acted Like It Went Into 4hi Once And I Drove Out On The Beach A Good Ways And Got Stuck. Luckily I Kicked It In 4lo And Drove Right Out Of It.

Thanks For The Info. I Will Pull The Battery Out And Check The Hoses First.

By The Way: 3 Button 2000 Zr2

I Found Out Buy Driving On The Beach. Most Times It 4hi Just Blinks And Goes Back To 2hi. 4lo Will Go In Most Of The Time. It Actually Acted Like It Went Into 4hi Once And I Drove Out On The Beach A Good Ways And Got Stuck. Luckily I Kicked It In 4lo And Drove Right Out Of It.

Sounds like the vac actuator under the battery

This is obviously a common problem. My 98 blazer is doing the very same thing. 4lo with engage but as soon as you shift into neutral is goes back up to 2hi. If i play with it a while i can get it to go into 4hi but the front end never engages. If i find the problem i'll be sure to post.