Front end humming... in 2wd

[Nsias1]

I have a 01' with about 187k on it. I had a ball joint and brakes done on it. drove it about 2500 miles since. About a week ago I put it in 4wd just for normal use it or lose it reasons. drove about 5 miles and put it back in 2wd. The next day I got up to about 50 and noticed it was humming, kinda like being in 4wd. it got really loud at 70+ mph. So I took an exit switched to 4lo, 4 high and auto 4. Drove it back to the highway and eventually (10 miles or so later) while driving 70+ the sound went away (also feels like a slight drag). then for the next 3-4 days it would phase in and out when above 50 still just having 2wd selected. Now yesterday it was mostly doing it driving about 100 miles at around 60mph. Today I even notice it at lower speeds 30ish. also gas mileage has dropped about 2 mpg. Any suggestions. I was thinking 4wd is not disengaged but no notice from the message center. Or a bad hub?