This happens only when you step on the brake? Or is it actually when you take your foot off the accelerator? If you were to put chocks in front of the wheels, start the car, and shift into Drive without touching the brake, would the problem occur? Or not until you step on the brake? What happens if you're driving along and you shift to Neutral before stepping on the brake to bring the car to a stop? What happens if you step on the brake while the car's in Reverse rather than Drive?
ATF+4 is fine. Dexron-VI is superior in every respect, but not enough so to warrant spending the time, effort, and money to go draining out your ATF+4 and replacing it with Dex-6. These transmissions are highly tolerant of crummy fluids; they'll even last a long time with an archaic, inferior-across-the-board fluid like Type-F, so don't fret about it. If you eventually have occasion to drain the transmission and torque converter, pick a reputable brand of Dex-6 (Mopar will even sell it to you, if you want).