Rattle with acceleration

[Sierra54]

I have a '99 Z-71 Sierra regular cab that will will rattle as if a metallic clicking sound is coming from the rear end. This occurs primarily under mild to heavy acceleration but will occasionally occur while braking. Does anyone have any ideas as where to begin my search for this annoyance. I am going to check for loose heat shields, differential fluid level, but would greatly appreciate any suggestions.

[Sierra54]

All of the heat shields seem secure, as well as the exhaust. However, when I removed the fill plug from the rear differential fluid started to come out. I just finished driving it 5 min proir, and have checked it cold with it below the fill plug level weeks earlier so I don't thik that I overfilled it when I changed fluids in January.

[Sierra54]

After doing a search on "rattle" I saw several similar coml;aints diagnosed as bad catalytic converters. After Pounding on mine and reproducing the rattle I believe I've located the culprit. Any suggestions on a quality replacement?

[superbee]

I got a Catco from Summit Racing and am thrilled with the sound and performance boost. Dynomax was 5 or 10 bucks more with a three week wait, glad I picked the Catco. What ever you decide to get, make sure you get a direct fit replacement, not universal, that is honeycomb/mesh type, NOT pellet type (stock style).