yukon xl power steering

I am new to this forum! I went in search of help and come across this forum! I have a 2002 yukon xl wiht 15,000 miles on it. I was out in the snow today (not in 4 wheel drive) when what seems like my power steering went out. I check the power steerign fluid and it was full but it was frothy/foamy looking when the engine was warm but looks fine when cool. Have to recheck this is the sunlight tomorrow to be 100% certain. If I am driving at normal road speeds it acts OK but still a lil harder to turn than normal but nothing like when i am doing slow turns. I basically have to stand up on the floor to get enough leverage to turn the wheel. There is also a whiney whirring noise at all speeds but it is a lot louder when turning slowly. If you could give any advice I would greatly appreciate it! Thanks!! Amy