This is the first time posting...so hope I'm in the right place to post this question I have.
2003 F150 FX4 offroad 5.4v8

A few months ago, I started to notice a slight buzzing-humming sound with little or no vibration at highway speeds. Now I am noticing the noise at speeds as low as 35mph and it is noticeable louder with slightly more vibration. Its hard to explain, but it seems to be more of a noise than a real vibration though. I read a post awhile back with someone talking about a four wheel drive issue where the system was not completely disengaged and caused a vibration issue -maybe some issue with a vacuum leak. I checked my four wheel drive and noticed it is not engaging and disengaging properly. Could this be a possible cause? I'm also suspecting a possible wheel bearing because the sound completely stops on taking corners to the left and doesn't change on corners to the right. It just doesn't sound like a wheel bearing noise though!

Any suggestions on where I should begin?