99 Chevy Silverado 2500 works in auto 4x4, but not when I select either 4 Hi...

[Eric]

...or Lo....? I got stuck today, tried 4 Hi and Lo, but only after I shifted it to Auto 4x4 did the front wheels engage. I can hear the engagement when I push the 4 Hi and Lo buttons, but it doesn't get to the wheels.

Thanks for your response.