I don't even know how to describe this

Okay this just started recently but at about fifty mph there is a strange noise, it almost sounds like somethings grinding, but its a soft grid and it only seems to be around that speed or higher. I know I need a new u joint for the front passenger side I was wondering if that could be it. Also to better describe the sound I put a flowmaster muffler on about two years ago and the noise resembles the low rumble that can be heard from the interior from the exhaust only louder. Any ideas?

While replacing the u-joint, check the unit hub/wheel bearing for roughness when turning or looseness. Hubs and u-joints are normal maintenance items that are replaced when reach the end of their service life, just like brakes and mufflers.