Friend of mine just called me as I was about to leave for a kids hockey game and asked. He's usually fairly savvy but said he bought a wrench to hold the pump blade or something or but was getting nowhere so he asked me and Ifigured I'd just post it here and he has a cell phone that can look over the forums. He has the 2002 Escalade with the 6.0 litre vortex. I figured it is probably pretty similiar to the Tahoe so posted it here.

Thanks if you can help... Gary

 

You can use a large flat screwdriver, and position it between one of the bolts, and the threads from the fan to hold the water pump pulley still while you removing the fan. Or you can go out and buy the tool spefically designed to hold the pulley. I am assuming you already have the flat wrench needed to get into the nut from the fan.
Here is a link for he tool set... http://www.sjdiscounttools.com/sch61200.html