ok guys I found some interesting information about these transmission and shreddin 3rd gear
http://users.tpg.com.au/tgrandja/images/Articles/4wd_gearbox_solution.htm
The info is based on someone who did rally driving with 323gtx awd, which have a similary gearbox than ours.
It seems the main problem is the bearing, the bearing sleeve can slip and rotate in the bearing groove, which wears out the case (caused by sideload). more wear = more play within the primary and secondary shaft. Please read the link to understand my explaination.