I changed my rear end fluid awhile back and reused the gasket since it was in good condition.
No parts stores at all have any part number for the 99up 10 bolt Rear end cover gasket.

 

The part number that I have on my receipt from my local dealer parts dept is 26066456. Rear end cover gasket for my 99, I assume that the 04's are the same.

 

I just bought a roll of cork from AZ and cut it to fit. No leaks 6 months after I did it

 

honestly from what i have seen every day rtv works best. although i think it is kind of rigging. i have used factory gaskets 2 times on my truck and have had one good leak. what i prefer to do is to put a lite coat of rtv on both sides of the gasket, you have the gasket for the peice of mond and the sealing power of rtv.