How many seconds does it take to engage Drive? On mine, which I've owned since it was a pup with 25 k miles, it has always taken about 2 seconds to engage Drive after I stop and shift out of Reverse. This is not unusual or abnormal. Two seconds can seem like a long time, and if you get in a hurry and hit the gas before the tranny has fully engaged, you'll get that "thunk" when it finally does engage, and is now under load. The fix is to take a deep breath and wait.
On the other hand, if it's taking 4 seconds or longer, that's a sign of wear and an impending overhaul, as I understand it.
How many miles on the tranny? Is the fluid level OK? How long since the tranny's been serviced?