I will try to explain this the best I can:
Rear Axle Spline (25 teeth) MALE END.
QDF5W differential (25 teeth) FEMALE END. They match perfectly. Here's the problem the rear axle spline will only go in a 1/4in into the differential. The bore is 1.362in (34.6mm). So here's the deal the rear axle spline will be need to be put on a lathe and smooth down about 3/4in to 1in and taken in about 2mm to fit perfectly. The rear axle spline has a sleeve that's preventing it from going in perfectly. Hopefully I explained this so you can understand.

 

Hello everyone. Well I met with Southern Gears/B-I-P and we discovered the Quaife is defective. The axle gears are milled incorrectly causing the axle stubs not to line up properly. I spoke with Dave K @Quaife UK and Racing Green Cars UK and they've agreed to refund my money and send another differential. You can watch on Youtube (Quaife QDF5W Defective). Jason @BIP will explain the exact problem.