Overheating, or undercooling?

I've got a 2004 Sierra 3500, 6.0L, Auto 4x4. The truck has a 9'6" Western V-plow and a V-box salter on it. All during summer months the truck never overheats. Once the plow goes on, the thing shoots up to 220-240, and if the plow is all the way up, it will actually get to the 260 mark. Question is, is it just a matter of airflow into the rad, or could it be the cooling fan clutch taking a vacation? The clutch is only $57, but I'd like to know more before I spend that money on the clutch - like how to test it? Thanks for looking!

Yep, air flow is your problem. You have to live with it with a plow or adjust the angle of the plow till you get more air flow. With my truck I had to lower the blade to get it to cool down. I tied everything else, new HD radiator, new HD clutch fan etc and that never did anything.

The only thing that you can do to protect the engine is to use synthetic oil. It will handle the heat much better than the petroleum oils.

And if you are still using Dex, get rid of it. Mine always ran hotter with it so I changed back to old green stuff. You aren't supposed to, but everyone I know with a GMC did it too.