Is a 1999 3/4 ton rear end compatible with a 2000 1/2 ton silverado automatic

SOURCE: Replaceing rear end cover (rusted

Fill it Up to the Bottom of the Filler Cap Opening. Should be 3 or 4 Bottles. Please Rate My Response! Thanks!

SOURCE: Rear end cover rusted out

The Previous Solution that I sent you was Straight from the Chilton Repair Manual.

SOURCE: Rear End in a 2000 silverado moning whenever I turn a corner.

I would suggest a rear differential service. When the fluid breaks down in the rear it doesn't provide the proper lubrication for the rear and you'll hear the "moan". Don't forget to buy and add the limited slip(posi-trac) additive.

As far as the rear end noise, There was a TSB released on GMC light duty trucks that says "Some owners of light duty trucks equipped with automatic transmissions may comment that the vehicle exhibits a clunk noise when ******* between Park and Drive, Park and Reverse, or Drive and Reverse... Whenever there are two or more gears interactin with one another, there must be a certain amount of clearance between those gears in order for the gears to operate properly. This clearance of freeplay (also known as lash) can translate into a clunk noise whenever the gear is loaded and unloaded quickly, or whenever the direction of rotation is reversed." It goes onto say "...Whilse some owners may find the clunk objectionable, this will not adversely affect durability or performance."

SOURCE: rear end gear ratio

you can count the teeth on the ring gear and divide that # by the tooth count on the pinion. An example is: ring gear 100, pinion 10 ...100divided by 10 is 10, expressed as 10 to one. (fake #) your diff is likely a 3 series such as 3.23 to 1. or a four series such as 4.11 to 1. If you have four wheel drive it is critical to match exactly. if not you can get better performance by changing ratio to something that better fits your needs. 4 series pull loads well, 3 series are better on fuel.(depending upon terrain)

SOURCE: how many guarts of 75/90 gear oil does a 1999

this is going to depend on a bunch of variable, engine, ring gear size etc. Try this site, you can pick all your variables.
http://www.amsoil.com/mygarage/vehiclelookup.aspx

If you don't know what size ring gear my best recommendation would be buy 4-5 quarts and then return the full one(s) that you have left. I have a 2007 chevy 2500 hd with the duramax and I think mine took just a shade under 3 quarts.