FLM aluminium tranny case

I'd try and work through Mike first before FLM. But finding the problem should be first order cause your case may be fine.
First thing's first, does the top shaft spin freely without the spur? Is no, the problem is deeper within the case.
You should be able to see the problem prior to assembling the other half of the case. If you hold the top shaft and diff asm perpendicular to the case sides as if the second case half was on it, do the gears mesh correctly? If no, you have a bad case. If yes, repeat but use the other case half. Measuring the center to centers with calipers and comparing is the best way to confirm this. You may be able to use half of the Al case and use the mating half of the plastic case to further isolate the problem. If you back all the screws on the case assembly does the binding loosen up? If so, you may have one too many washers in there or your case is too shallow for some bizarre reason, squeezing the gears. Whatever the reason, you're more likely to get better support when you can talk intelligently about what the problem is. Good luck and don't give up casue a good Al case is sweet love when it fits.

PS. Make it the real deal with the Al diff Mike sells also. You'll never need to deal with a busted diff gear again!