Does it leak? Or is it posing a problem that you've found?
All diff cases will exhibit some sort of "slop" with the diff outdrives. Since these aren't super high end components, the tolerances aren't going to be very high. As long as it seals the oil inside the diff case, i'd keep running it. Depending on the thickness of the oil, surface tension may be enough to keep it in.
You can improve sealing by using some thicker grease on the diff output and inside around the o-ring (Tekno even says so in their assembly instructions) when assembling the diff.