When I shift my Tracker into drive sometimes it.

When I shift my Tracker into drive sometimes it just sits there and won't move, what could be the problem? Also some times when I shift my Tracker into reverse I feel a thump, what could that be? One last thing, when slowing down to make a turn or when stopped at a light or something and when I go to accelerate my Tracker gives off a thump. As well. Ant clues as to what the problems could be?