As you know, sometimes it doesn't take much at all to throw everything out of whack. If you have any items which have play but not much make a note of it as a couple problems may be complimenting each other.
Pull the calipers off to ensure that the glides are not sticking.
If it only happens under load I'd add checking the subframe, diff, half axles and transmission output shaft to the list.
Oh, and check the PS rack.
That's all I can think of...
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