shapeways should give you tolearances for joints.
But f you make the balls about 0.5mm less diameter than the socket, you should get a good fit.
But check with shapeways - you might get away with a smaller difference.
I think the minimum clearance for sls is about 0.1mm (so 0.2mm diameter difference) - but check first.
Oh yeah and as they use sls - you can actually send the model as a complete unit.