4WD Stuck

I have a 1997 4WD Sierra 2500. It slipped out of 4WD and I cannot get it back in. The control lever in the cab will not move. I spayed all joints with WD40 and it still won't budge. I remove the plug and there is no fluid within reach that I can feel in the pumpkin of the 4WD unit. What fluid and how much should I add. Is there a way to free this up? The truck was stuck in 4Wd and was working fine when I bought it. Appreciate any discussion on this.

Thanks

BLE