Thanks a lot for all the comments!
I've cleaned one outdrive and diff plate used some tamiya AW grease and it seemed hard enough to get it to slip by hand.
Tried the other one but that seemed fine even harder to slip so didn't clean that one.
But when i assembled it and tightened it i noticed the diff action got tighter when i turned the srew side outdrive right and looser when I turned it left.
Removed the thrust bearing and it looks dented. Think i've overtightened it at some point.
So I think that's probably what's causing all the problems cause the screw isn't free to turn with the other outdrive.
Diff balls are 3mm and are running perfect in the front.