Vibration from driveshaft

[ScottyB]

So I replaced my transmission with ond from an outback with the 4.44 final drive. There was some problems with tranny fluid leaking due to old hoses, but now that's all fixed and the tranny seems to be running smoothly.

So this is what happens. The car's in the air on 4 jacks and when I put the car in drive and give it some gas the front tires start, then at some rpm the rear wheels engage the driveshaft jerks violently and bangs into the bottom of the car. After the initial jerk dampens out a little, the vibrations still continue. Everything I've looked up talks about the staked u-joints so I checked them. There didn't seem to be anything obvious but LAN said in some thread that you can't always tell until you take it out, so i did. There seemed to be an uneven spot on the u-joint closest to the rear diff. All the other rotations seemed fine, but I figured this uneven spot was causing the problem. I went to get another svx drive shaft, but the mechanic said mine seemed fine. He said it's normal for driveshafts get a flat spot like that and that the svx driveshaft he has looks and feels just like mine. So I put my drive shaft back in and the vibration's still there.

So what could it be? (And it wasn't doing this before I switched trannys)

Should I get the driveshaft tested by a driveline shop?