Since I have my 78 up on jack stands for the brakes, I figured I would replace the rear differential oil.  I've read here various comments on this, but have not really figured out if my 78 has a drain plug on the rear diffential, or if I have to suction out the old stuff.  There is a bold with a red tag on the differential, but not sure which that bold is.

Oh yeah, how much oil after I add the additive should it take?

The bolt with the red tag that you're referring to is the drain plug. It should be a 4 sided plug on the passenger side (as opposed to a 6 sided bolt). Suction is the way to go. Your local auto supply should sell a suction tool drain it.

When you refill, you'll want to add a 4 oz bottle of GM positraction additive and enough gear oil to get you about 3/8 of an inch below the drain plug hole. Usually about 1 1/2 quarts. Fill slow and measure as you fill . Make sure you get the correct weight gear oil and take some time to clean the differential around the plug area PRIOR TO REMOVING THE PLUG so you don't get any road grit in the fill hole.

Good luck.

Paul

Thanks Paul.

That's what I thought, just was hoping I was missing a hidden lower drain plug.

Any idea what size wrench that plug takes?  13/16 is too big, 11/16 is too small, and 3/4 doesn't fit.  Having a Masters degree makes me think that my math can't be that far off, and having that the Vette was made in 1978 makes me think it probably isn't metric....

When faced with a problem such as yours do what I do pick & up the crecent wrench..just keep it tight

Alan

All set.  Turned out to be 3/4" square bolt.  I was trying to use a 6pt 3/4 socket on it.  Tried again with a 12pt socket and fit like a glove.  Can't wait to see if that f*!k'n noise is gone now. 

Thanks guys!

My posi-clutches were chattering really bad. I drained mine with a pump I bought at Boaters World. The pump worked like a champ. I went with one bottle of additive and 2 quarts of GM rearend lube. I haven't had a bit of noise, chatter, or trouble since. Hope it works for you.
Scott

The reason the plug is in the location it is, is because you should put the additive in first and then fill till the fluid just starts to come out the hole.  This will ensure you have the correct level.  Just make sure the car is level.  If you attempt to fill to 3/8ths below the hole (I have no idea how you could measure it correctly) you will not have enough fluid and you may have clutch or bearing problems later.