Wierd Vibration From Under Floorboard

So here lately, I have been getting this strange vibration-like feel from when i put my left foot flat on the floorboard while driving. It seems to only happen at higher speeds. (over 45 mph i would say) I recently re geared the rear of my jeep but still have the front DS connected to the non re geared front till i can do it, and am wondering if maybe its the culprit. Also seems to me like it could be a bad u joint, because whil eim driving, I will feel around the different spots of the floorboard, but I only feel it really in the middle right over where the driveshaft sits under me.

Remove the front DS and road test it.

could be a u joint! check the rear yoke sounds like it might be loose! mine used to toss u joints quite often when it did it would make a noise at 40 and vibrate at 45! check all your joints!