Worn spider gears shouldn't make noise riding straight down the road right? Only when turning? Because they aren't turning on each other going straight correct?
If both tires are exactly the same diameter and pressure, and you are going dead nuts straight, the spider gears shouldn't be turning.
EDIT.... I have an idea for a test..... with the wheels bolted on, jack 1 rear tire off the ground , the other tire on the ground and chocked. Put the trans in neutral and rotate the tire. If the sound is still the same, switch and do the other side. If you hear it the same on both sides, it's probably the diff. If you only get it from one side then probably a wheel bearing, as Rusty said.