from what you have told me the problem is not in the spider gears.... If it was the spider gears it would make the noise when u turn one wheel...so it's safe to say the diff action is fine.
And by the sounds I don't think its the axles or hub carrier bearings, because it would happen all the time not just when both wheels are turning..
so all that leaves is..
motor or motor bearings
pinion to spur
slipper to idler gear
idler gear to diff ring gear
gearbox bearings
debris
gearbox housing screw in wrong hole or incorrect length
just check everything over making sure the gear teeth are nice and smoothe with no pitting or debris stuck inbetween the teeth
and check to see if u still have the binding issue with the motor removed....