Loud annoying sound coming from underneath.

When I hit 35-40 mph there is a loud kind of throbbing sound. It doesn't really vibrate it's just bad harmonics. I replaced all the u-joints in my front driveshaft yesterday and that didn't fix it. When I remove my front driveshaft and drive the sound is gone. I'm leaning towards something in the t-case being bad but I don't know. Has anyone had this and what did you do? Thanks