2002 Ford Explorer

My Explorer just stated bucking upon deacceleration. This problem is most prevalent when driving along about 45 MPH in either 3rd or 4th gear. When you take your foot off the gas pedal the car will start bucking after a short time. I had the car in the local garage and they replaced the RPM sensors on the transfer case. However, this did not solve the problem. The engine idles and accelerates fine. No engine check lights come on. Do you have any idea what is causing this problem? Thanks, Phil