Q: Torque spec for transfer case drain plugs

I need to change the fluid in my 99 OBS Tahoe's transfer case.

Both the NPT drain plugs and the case are looking mighty soft. Can any of 
you techs tell me how tight to tighten the plugs?...maybe a torque spec
from 
the guides? I'd like to just set the torque wrench and be safe.

I have the New Process two-speed push-button model.

Thanks in advance.