I have just built up my gear diff. It has gone into a complete gearbox (so I have the option of gear or ball). Mine is tight, but nowhere as near as tight as any of you guys are complaining. When holding the gear box I can spin the spurgear fairly easily. Just a thought but how have you guys shimed the diff? I put two either side as per the ball diff and noticed that the bearing where quite a tight fit onto the diff and had to push them on quite firmly before they seated properly. Just a thought for you to check that the bearings have pushed on all the way.
HTH
Scott