Your air bag sensor is on the tunnel,no wonder it went off.
I did a diff for a customer recently who had swapped in Used diff...which ripped it's crown wheel apart.
The reason the diff was changed in the first place was because the owner heard a noise and his alleged ( former) mechanic told him the diff had gone.
It wasn't the diff making the noise but the flex disc,just like the first one in the thread.
It was in a 6.3 which eat discs anyway.
As he coasted down a hill on a city street,the flex disc came apart completely and took the rear of the gearbox out,plus the crossmember and the shifter rod
The drive shaft then hit the road and bent backwards taking out the center bearing.
The speedo cable was ripped out the housing.
He repaired it all himself .Luckily the finny uses the same rear housing etc on the tranny so it only cost a few dollars to replace things.
Then the diff went..