I have a 2004 Ranger 3.0 V6. Almost every day on my way into work there is a slightly inclined road with a speed limit of 45. When I come to the road and begin to accellerate it starts bucking when it seems to be trying to shift to a different gear. Sometimes it makes a quick vibration like those white lines you run over when coming to an intersection.

Already replaced Plugs and wires.
Any suggestions??? Thanks!