2008 Toyota 4Runner Tranny problem?

8/29/09 - I just purchased from a dealer an 08 4 Runner 4x4 with 7000 miles. I test drove it approx 15 to 18 miles and noticed something in the drive line when I came to a stop. It felt like a slight tap from a car behind me. I only felt it once and chalked it up to it not being driven in a while. After buying the car I left and drove 300 miles on the highway. It performed perfectly on the highway. But when I pulled off to find a hotel and was in city traffic and it started doing it much more frequently. When I come to a stop, it feels like I put it into gear even though I am already in gear. It seems to happen more often when I come to a quick/hard stop. I test drove a 2005 Lexus GX470 1 month ago and felt the same thing. Has anyone had or heard of a similar experience?
Desperate for some answers.
If it is important, the car was never titled and purchased as a new car with new car warranty.
Many thanks.