2000 Ford Expedition thud/lurch

We have a 2000 Expedition we bought it new and all the sudden it makes a lurch and thud that can be heard and felt at low speeds it is always in the parking lot or drive way when creeping at low speeds, also in reverse it is intermitant and usual when the vehicle is heated up. I checked the U-joints 3 times and they don't appear to have slack the tail shaft has about an 1/8 inch side to side movement. I feel like it comes from the rear and is in the drive train you can hear the bump and then it feels like you ran over a rock or something. I changed the fluid and filter on the trans and checked the level but it still does it.