i've had issues with my r200's too. ive fitted several different r200 clsd's in my s12, and the axles dont clip in securely on most of them

comparing all the axles/diffs i have here, i seems that the spacer in the centre of the diff, between the ends of the two stubs, varies from diff to diff.
i found that the distance from the circlip groove in the stub, to the end of the stub shaft varied a bit.
i ended up just linishing off about 1mm off the end of my stubs, and now they fit and stay in no problems...