RTV is sufficient if applied properly and given time to cure before refilling.

 

I used a pretty thick paper gasket and put a little rtv to help it stay in place and just to add a lil better seal

 

Did you add the friction modifier, since you have the 3.55 LS rear? Or did you buy the oil with the modifier already added?

I changed my rear diff oil a couple of years ago and use the Valvoline, but bought the friction modifier and added it myself.

 

It already Had the friction modifier in it so I didn't bother adding it