Well, if the splines of the driveshaft are notched, that wouldn't be a problem. I've only seen the ones that aren't notched, and have a plethora of combinations not suitable for experimentation.

The balance weight is almost certainly the culprit.

I should note, however, that on my own car, I had a rumble such as that, caused by a failure to properly tighten a few of the output shaft-to-driveshaft bolts. With new universal joints installed, the new shaft should be quiet regardless of change in angle. Unless there was a problem installing them, I'd say that the balance weight was the problem. Try putting a few hose clamps with their screw ends where the weight would've been (so the weight of the screws is where you need it). If it works--you've got the answer.
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1992 940 wagon, 72k
make people envious; smile often.