do you need to drop the rear to swap gears?
or to install a posi unit?

 

No.

No.

But I will say I've done both things both ways; and it's a whole lot easier to do when you can stand up, or sit down on a chair, or whatever, rather than laying on your stomach in a pool of rear end grease. But if you have a rack, it isn't near as bad as doing it on jack stands.

The gear swap is a whole lot worse in the car than the posi.

 

considering how easy it is to R&R the rear i do them with the rear out, but you don't have to.