DML FAQ : Transmission and Gears : Do I need a gasket for the diff covers?
RTV is what I used and what I've heard most others say they've used. The Ultra Copper might be overkill (I just used the "regular" orange stuff), but it won't hurt anything.
Incidentally, if you want a chrome cover for your rear diff, I noticed that Mancini Racing has them for $27 for the 8 1/4 or 9 1/4 and that includes a gasket. Chrome bolts at an additional $14.