1999 Chevy Truck Help Silverado

I am getting a loud humming sound out of the back of my truck, so far I changed all of the U-Joints and had the rear gears checked for any failure and there was none. The noise seems to be consistent on/off the gas with the exception of maybe a little less in between the slop of drag/push as you let off the gas. Someone told me to change the pinion bearing and outhers told the problem is a rear wheel bearing. On Jack stands it is verry difficult to pin point the actual area where the noise is from. This issue has been going on for about a year - and to me it dosen't seem to be getting any worst.