The newer axles got synthetic gear oil, but I wouldn't hesitate to run regular 80w90, if you've already got it.
You should be able to ID the gasket you need just by looking at the cover, while on the truck. As far as swapping covers, if they fit, they fit. I think they will. Looking at Rockauto's offerings, the fitment guides look incomplete, to me. If I look up a 73, the fewer parts they list often go up to the 2000s, but'll skip 87. If I look under 2005 they only go back to the 80's. I think it's just screwy listings. Covers don't really need to change to accommodate a new vehicle platform. Maaaybe the accommodation for the brake lines changed?
As I mentioned elsewhere, the gaskets I've used for the axle shafts were a funny paper. As long as there isn't a significant thickness difference, I'd think either would work. Some people skip the gasket and go straight to RTV.
New GM covers are available, too. I think I remember someone in the last few months buying a new GM cover that was cheap/crappy, Though. I got mine several years ago on ebay as a kit.... cover/gasket/bolts.
Bolts.... my 99 still uses SAE cover bolts. I'd guess your 2005 does too, but I'd suspect you could get something functional from Menard's or TSC, either way, if you needed to.
A wire wheel on a angle grinder makes quick work of old gasket material, just don't get any stray wires stuck in you, or inside the differential. I haven't been able to find a wire wheel that doesn't throw wire.
What made you decide to pull your axle shafts? That oil looks really brown.