it's the back tailing out, the front sticking. means your back is the problem, not the front.

shocks too stiff and springs too stiff can cause that (or as mentioned those long type shackles, but i never drove one with them, so i don't know)
my car is doing the same on very rough roads, once even with steady throttle. i have 5leaf mid-eye and those gas-adjusts. the 5 are a bit too stiff, but i'll change the shocks in mine should i ever get around to that

 

If you haven't done the Shelby/Arning drop yet and you really are experiencing oversteer that you aren't causing, you need to fix what's making the oversteer happen before going any further. Either adding grip up font (what the Shelby drop generally does) or taking grip away from the rear (what adding a rear sta-bar or swapping an existing bar out for a stiffer one does) will only aggravate any oversteer condition.

We still need more information.

Pay attention to things like how much throttle you're using, whether the freeway on/off ramps really are constant radius (they probably are not), what your line through the turn is, whether you're getting impatient as far as adding throttle is concerned.

Check your front and rear tire pressures. Are your tires and wheels the same, front vs rear"? Rear suspension bushings OK? Shackles not loose?


Norm