Ron's description:
In these pics you will see the aluminum driveshaft from a police Crown Vic that was installed.  It requires the re-use of the civilian slip yoke, as the slip yoke from the police CV is a bit different.  It also requires the purchase of a carefully machined aluminum spacer to be used at the rear of the driveshaft (made by Fabtech, purchased from adtr.net) as the aluminum driveshaft is about 3/4" shorter than the civilian driveshaft (because of the slip yoke differences).  There are really good writ-ups on this swap.  So I guess the question is....why bother?  Well, it sheds about 14 lbs from the weight of the car and reduces the rotating mass, thereby allegedly reducing the time to get to desired speed.  I don't know if all of that is true, but it does reduce the mass of the vehicle.

As far as the aluminum differential cover is concerned.....it allows the shedding of heat much more efficiently and faster than steel (probably mostly because of the fins).  Also, the new diff cover has drain and fill provisions (the steel one did not).  The aluminum cover also has pre-tesioning bolts near the top of it (see pics) which apparently allow you to pre-load the gearset to prevent any movement, which in turn causes additional wear.  The last reason is......I wanted to get that old gear oil out of there.  I am assuming that it was the original oil from the factory, and it has certainly done its job all these years.  I found no ground up metal bits in the oil or laying at the bottom of the diff case, but the oil was dark green and man, did it ever have a strong odd odor to it.  Whew!!!