Hi There,
You really should have a gasket under your differential cover, I always have as I've never found just sealer of any type adequate, if you have a standard 7.5" rear the Fel-Pro part number is RDS55072, I like to use Permetex Aviation Form-A-Gasket along with a gasket which is a thick liquid that has a brush to apply it, I use that on both sides of the gasket and have never had one leak in many years of working on cars, also the correct oil level with anything that has a side fill oil plug be it Rear Ends, Transmissions or whatever is dead even with the bottom of the fill plug hole with the vehicle level, also in fourty years of working on vehicles of many types I've never encountered a sealant that I could.t break loose by driving a couple screwdrivers, chisels or putty knives into the parting line, you just were'l trying hard enough, hope it helps!! Good Luck, KillerDave