Rear End, Differential

I took my Maro to a shop today and the said the rear end is definatly going out. Does that mean the differential is going out or are they the same thing? Also why they are working on it I was thinking about swapping gears to 3.73 gears, would this be worth it. I do mostly city driving but occasionally I will hit the freeway.

I would interpret that to mean that the differential is going out. However, don't act like you don't know around a mechanic unless you trust them, sometimes (not always) they may try to take advantage of someone who doesn't know better. With regards to the gears, it depends on what you have in there now... Go to this site and check out the gears they mention: http://mods.firebirdv6.com/

You should have an open differential if it hasn't been changed, you might as well do a rear end swap if you're going to replace it. That way you get a limited slip differential and disc brakes.