You are troubleshooting the best you can. Good common sense approach. You will probably smile when you find it. Hope it's not a big money thing. You know Ujoints need to be "phased". Lotta goofy things you can do in the name of elimination, like go down a hill with the drive shaft off. Coast with the engine off, and as you're doing, jack stands. If it still does it on jack stands, that may narrow it. If it does not do it, that doesn't mean anything except that it doesn't do it on jack stands. (considering all the things you say you have eliminated that roll on the ground tires/wheels)
