Rear of E55
 

When I accelerate fast the rear end initially seems to fishtail. What does that indicate? Tires or suspension.

Probably bothbut id bet ur rear subframe bushings are bad

If you tell us a little more it will be easier to help you, remember we're not there in the car! ESP is off? Have you always had this problem or is it new to you?

These cars have an open diff so the ESP has to intervene. Basically it will brake one wheel then the other as difference in speed the two rear wheels are spinning gets too much! This used to the rear of my car wallow to either side when accelerating in a straight line!!

My rear sway bar end-link was loose and I would get a lot of lateral sway when accelerating hard even with a LSD installed!

Check the rear tires, maybe you could compare the amount of tread left on the tire and compare it to the fronts! Keep in mind the rear tires will be more worn on the inside lip of the tire, because of the 'camber'.

How many miles do you have on your car?

OK. I just put the car is Sport mode. No other changes to typical driving. I checked the tires and both have equal tread and no difference on each tire from inside to outside. They are relatively new tires. The car has 75,000 miles.

I have yet to get under the car to check the condition of the bushings visually. The car is only about 6 months new to me. It needs new brakes and that is when I plan to check out the suspension. I will look closely at the ones suggested here. The comment on the open differential and ESP may just be the answer as well since I am not knowledgeable in how the car should act.

Thanks for the thought so much and I will check out these suggested items.

exactly what he said differential bushings check them too