Loud whining from front end ZR2 Pick-up

[denisrd]

When driving {2 wheel high} on highway 60-70 MPH for an extended time, 20+ miles or so. Out of nowhere comes this whining noise from the front end, sounds like a turbo prop plane. If I switch to 4 high, the noise goes away instantly. Almost signs like the 4 wheel drive is trying to engage and not meshing properly. Any help would be appreciated.

Thanks all.

[newdude]

I believe those are vacuum actuated front diffs. I'd check all the vacuum lines to it if there was an issue in the lines or the vacuum actuator, it could be trying to engage the front axle on its own. There is the actuator, a solenoid valve and vacuum tank as well as lines.

[00Silverado4x4]

I don't have specific advise but the first thing I do when I hear or feel something is to check fluid levels and conditions. That will rule out a few things to start off.