I doubt its the pick ups at all. just check the resistance of them with your multimeter. I assume that you haven't got the ground and hot wires mixed around in one or both pickups? That wouldn't cause a grounding issue but it would cause some hum.

It looks like there only one wire going to the jack? or is there two encased in heatshrink? If so, that would mean you have the earth wire running from the master tone to the jack instead of the earth coming bridge volume pot. I suppose it shouldn't matter (if everything is grounded well) but do you have complete earth continuity between the ground lug on the jack and the back of each pot and the earth lug on the switch?

The solder joint on the earth lug on the bridge volume pot could be a little bit better. It may be fine now but could cause problems down the track in a hollowbody.

Sorry thats all I can see mate.