I've no experience in buying a used pickup, but I would assume the worst time to get a good deal would be right before/during the snowy season... though IIRC you're in Arizona, so that may not be applicable.
YRC - I'm in Arizona. We generally get snow every year in southern Arizona where I am, especially in the mountains. And it can be a big deal because no one knows how to drive in it. About 20 years ago I had a trip to Colorado planned for Christmas that I had to cancel because we had about 3 inches of snow, but that closed the roads for a few days - no plows, and the cops are paranoid/know that the sight of white makes people crazy here! There were major snows in Colorado but the roads were open. I could not get out of town here though.
So maybe it's not a major influence on 4x4 prices. But I think Tightwad was on to something with hunting season!