I heard that I could use my mini-vac pump to suck the old differential fluid out of the differential (thru the filler hole) instead of the major rear-end disassembly normally required. I was guessing that the differential would be so crowded on the inside that it would be hard to ensure that the tube was getting to the bottom and getting all of the old fluid out. Anyone have thoughts on how best to do this and any concerns about taking this approach?
Thanks!

You can get most of it with the pump. The disadvantage is not really knowing what might be laying in the very bottom of the differential (filings, spare parts, etc). You can tell how much you got out by measuring it and compare the quantity to your capacity chart (assuming it's full when you start). Don't forget to use the GM posi additive when re-filling.

another option is drilling and tapping a drain plug..

many rear end shops do this as SOP...

its pretty easy...

one of the vette places sells an idiot proof kit to do this.

I've always used a suction gun to remove and replace my fluid.
Drive the car around for a while to warm up the fluid and just about all of it comes out, flush out the gun and use it to "squirt" in the new.
The drain plug is an excellent idea, but it makes so much sense, it would probably drive a purist nuts!