The axle bearings in the rear end will growl much louder (when bad) when you load the truck in its direction. To check, drive straight at about 40 or higher, turn fairly sharply in one direction and see if the noise increases by quite a lot. Then do the turn the other way to see if the other side is the problem. If there is no significant difference in the noise level it is not axle bearings. Bad pinion bearings will result may resullt in sloppy pinion condition but not unless really bad. Drain the oil and see if you have metallic stuff in the oil. If you do then every bearing in the assembly is going to have to be replaced, including the axle bearings as I believe they are not sealed units, but oiled by the rear end oil. Another spot to look is the carrier bearing. The 2 piece drive shaft is cradled in the middle at the u-joint by a rubber suspended bearing. These things are right out in the slop when you are driving and make one heck of a growl when dried out. Most factory carrier bearings are not provided with a grease joint so they eventually dry out the factory lube and go to pot.

My bet would be the carrier bearing..