4WD trouble.....

Maybe someone can help.........I have a 2002 Chevy TB. For soem reason the 4WD does not work. I recently took this vechicle to a sand pit to test the 4wd.....When I put the TB into 4L it started to climb the hill then I heard a clunking sound and the 4WD disengaged and only the right rear wheel was spinning. After testing 4H and L several times afterward I can hear a spinning noise coming from the front wheels....... Almost like something isnt enganging. No service lights come on. Someone told me it could be the acuator.

Does anyone know what this might be and is this a costly repair?

Any help is appreicated

Thanks,

Fireman