You can rebuild the differential for that much. If it is only leaking from the cover, you need new gear lube and a gasket which totals around 20 dollars. Add about.5 hours labor. If it is leaking from the yoke, you need to replace a seal and crush sleeve. Parts are around 25 dollars and figure.5 hours labor.
WHY DID YOU GO TO A BODY SHOP? That is a job for a mechanic not a body tech.