Rear end
 

How do I know if my 8 3/4 is open or suregrip?

Block the wheels and jack up one wheel. Try to turn it If it turns easy it's an open rear. If it is hard to turn it is probably a sure grip

The more sure fire way of checking is to pull the diff and look at it. Even worn out sure grip units can act like an open diff.

Raise both back wheels and turn one wheel. If they both turn in the same direction, it's sure grip. If the other wheel turns in an opposite direction, it's open