2006 f150 fx4 wining sound 60 to 70 mph

i have a 2006 f150 4x4 crew cab and i just noticed that a wining sound starts about 60 to 70 mph in 2 wheel drive, sounds like its coming from the front i checked fluid in the front differental and it was full does anyone no anything that i should check, the truck has 89k, i dont think it would have a bad wheel bearing.the sound has got louder since it has got cold out.thanks for any help