New to me 96 5.7 seems haunted

low mileage find, 5.2 133k, everything seems fine but when I turn into a parking spot and turn the key off about 20 seconds later it feels like someone is pushing on the vehicle, it just rocks back and forth a few times. Something is storing energy and releasing it. I'm going to change the transfer case fluid a couple of times to see if that helps but has anyone else experienced this? It's very spooky.