Transmission problem?

I own a 2009 ford explorer 4wd 5 speed auto with only 44,000 miles when I drive using overdrive just before it shifts into overdrive it bucks and when I slow down below 50 mph before it will down shift to 4th gear it bucks. I have had the suv to a ford dealer repair service 2 times and they ran a diagnostic computer check on it both times and found nothing. can anyone tell me what I need to do next?

I think there is a good possibility that the torque converter lockup mechanism is not working properly. Try unplugging the electrical connector for the lockup mechanism (located on the side of the transmission case, IIRC?), and if the symptoms go away, then you have found the source of the problem.

By the way…your tranny should have had its fluid and filter changed at least once already. If this has not been done, I would strongly recommend that you do it in order to prevent other types of trans problems in the future.