Outer Limits 4x4 Board

Hi there could someone help me please. ive been told that the pinion gear in my front diff has collapsed. Can anyone tell me the best way to have this fixed, and or th cheapest way please.It got water in it which forced the grease out. is it fixable or is it better to replace it

probly best to go for a second hand diff head depening on ratio they start at 250 new zealand dollars. or i think superior enginering dose aftermaket cromoly pinion and crown wheel for 400 much stronger