Epilogue.

During last snowfall I intentionally stucked in the snow to chceck how does rear axle do it's job. Everything is fine, because of totally lack of adherence only one wheel was spinning, after diff lock, two of them were spinning at the same speed as it should. Locker is fine.
As a curiosity, few days later i did some "after snow mud offroad" and stuck again but i noticed that half frozen mud has a lot more grip and both rear wheels were spinning. It follows that mud isn't best environment to check if locker is fine.