Parts cost about $400 and that is for the better stuff; About three days labor for one mechanic @ $100 an hour is $2400. Get another estimate from a reputable shop.
WHEN THEY REPLACE THE REAR SEAL, BE CERTAIN THEY REPLACE THE SEAL AND THE MOUNTING PLATE IT SETS IN!!! NOT just the seal.
How old is the coolant? How old are the plugs? With the valve covers off it is a good time to replace spark plugs and wires. You would need to add that cost to the above figures and about three hours of labor.

PS. You may as well change the differential oil. They will have it up on a lift, so this is the time to do it. They oil from the factory is not good. If that is still in there it will be very black and strong sulfur smell; Replace it with full synthetic.