Tom:
Digging into the internals of differentials may be more complex than you realize. Why are you replacing your diff? Old one making noise? Want to switch from a 4.56 to a 4.10?
You can't just unbolt the old ring & pinion, install a different one, and be on your way. As ronsbackyard wrote, even if you could, it would be infinitely simpler to do it on the bench. Look at your rear axle assembly - the only way to get inside it is to unbolt the two steel tubes from the aluminum center section, and you couldn't possibly separate the tubes with things still mounted to the car.
But beyond that problem, there is the issue of setting up the R&P. Read that section of your Alfa repair manual. There are many measurements that have to be taken, many shims that have to be inserted/deleted in order to get the correct relationship between the R and the P. Practically speaking, it is an impossible job for a home mechanic (we don't have the measuring tools or a supply of shims), and a prohibitively expensive one to pay a pro to do unless you are building a race car or restoring an 8C2600 (it just takes too many hours). Swapping a complete rear end assembly IS something that you can do in your driveway.