Back axle gear sings…

Hello guys, after I discovered three weeks ago that the differential (without lock) of my omi at the front simmering oil saute I drove right into the factory instead and had it sealed. The day before yesterday then started this singing, thought first of a wheel bearing. So right back to the workshop, up to the stage etc. The mechanic said after various tests that the HA gearbox was defective and it had to be replaced. So, call Opel and ask for price from the new en Diff. Had almost gotten a seizure when I heard the price. Did they want to say and write 3450 euros. So I don’t buy a part that costs me more than one of my cars. What should I do now??? Did I look at i-bäh and also find what do you see here Do you think that’s good? Or does one of you still have such a part at home? Please for help!!!!!!