How to resolve a twitchy back-end in 1/8 buggy?

[VintageMA]

The back-end of my XB8 is very twitchy and I am looking for a way to calm it down a bit.

What is mean is that it's almost like an oversteer. When I am running on pavement it isn't a problem at all, but when I am running on grass and there is tons of traction it's almost like there is an oversteer and the back-end just wants to come around too quickly for the turning of the rest of the car. I was studying it a bit this past weekend and you could see that either direction you steered the back end really wanted to swing in that direction and in the grass would tend to grab and hop a bit (my description of twitchy) and generally flip the buggy if you didn't correct for it.

So I an basically looking for a few possible tweaks to mellow out the back end a bit.

I was thinking that standing the rear shocks up a bit may work.