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I have a '79, which means my car has a different rear end than yours. So, I don't know if yours has a pinion mount bolt & bushing or not. However, if yours does, that would be a good item to inspect.
I have a new pinion mount bolt & poly bushing on mine, and I still notice a slight rear end squat when I put it in gear, but nothing like it was before with the original bushing (what was left of it).
How much of a squat do you have? What is your idle set at? When mine is as much as 100 RPM too high, I get much more of a squat than when it is set at 800.
Definately check the pinion mount bolt and bushing.
We lovingly refer to that as the squat. Every C3 I have owned has done that and I kind of like it. When someone sees it do it for the first time from the outside while you're driving it makes a real impression. Kind of like when someone stretches a rubber band between thumb and fore finger. You know that thing is about to launch and you get all flinchy.