recently noticed my wifes 2000 expedition 4.6 2wd has a weird bump/lunge at low speeds it sounds kinda like bump on the tire about every one revolution of the tires and jerks forward and then back kinda like when you shift a 10 speed bike and the chain slips for a second until it engages this happens on when going under 10 miles per hour like in a parking lot or in the driveway but it is a real noticeable thud you can hear it and feel it. I looked at the u-joints and they seem extremely tight it has 75,000 miles on it. I did notice the output shaft has about a 1/4 inch play up and down when you shake it but the transmission shop I use said that is normal for that vehicle. I am not sure what the deal is I just hate replacing parts to fix it if I am not sure what it is to begin with. Any ideas would be appreciated.