I think the 2 hours will be fine, even on Thanksgiving Sunday. My reasoning is that the biggest thing to worry about is the Customs & Immigration experience, and Thanksgiving is very much a domestic travel-focused holiday. Once you clear the terminal will be busy/crowded/chaotic, but the line/wait for clearing should be at or below normal levels, and the 2 hours should be enough time.

Is there a backup option for getting IAD-PHX later that night??