Transfer box woes

Happy Christmas to one & all, i have the following problem with my 300 tdi transfer box & i hope somebody might have an answer. The other day had to use the diff lock, light came on & all appeared well. When it came time to disengage the diff lock that when my problems started, the lever for the diff lock suddenly had a lot of slop in it & i could not disengage the difflock. In the end i manually released the lock from underneath. When i got home i checked the linkage all seemed ok even though there is a bit of play on the top joint. Removed the top of the transfer box housing & found the small white nylon bush at the base of the selector lever had popped out, i pushed it back in & tried the difflock, again it popped out. Can anybody tell me whats going on as i cannot see if there should be clip to prevent it moving or can i simply hold it in place with a cable tie. Please anyone help

should have a circlip holding ball

Thanks for the answer, will strip down again & hopefully fix for good. I think it happened due to the slight wear in the top linkage & the effort to engage the diff lock. Hopefully should be sorted soon, again thanks.