1988 F-150 127,000 miles Shakes when slowing down.

My truck was shaking quite a lot and I determined that my ball joints needed to be replaced. I replaced them all and the truck drove great. For 4 miles. Now the truck shakes like hell when I down shift or just slow down. Once I've slowed down, not stopped. The truck will continue to shake until I have stopped. It doesn't do it when I accelerate or at cruising speeds. Just when I slow down or down shift. I don't even have to use the brakes to make it shake. I was leaning towards possibly a bad u-joint. When I bought the truck the u-joints had been replaced along with the drive shaft and a rebuilt transmission. Help would be much appreiciated.