[Msquared]

SAI (steering axis inclination) on the rear is...not exactly a thing, I think. It literally only matters up front, where there is actual steering going on; and I think it's only accurately measurable on an alignment rack by moving the steering through a range so the machine can measure the change in camber as steering angle changes. With a non-steering suspension, I don't know how the machine could possibly measure SAI. Now I'm not the Einstein of alignments, so maybe there's a way I don't know about, but I can't imagine what it would be. So I'm guessing a rear "cross-SAI" (a an alignment parameter I've never even heard of before) is a misinterpretation of something. Besides all that, even if a knuckle is in fact jacked, that shouldn't make the alignment change after you leave the shop. It's true that a bent knuckle could throw SAI off, but then it should also throw off rear toe/camber/caster as well. If they were able get those settings into spec and pretty close from one side to the other (the "cross" measurements), then the knuckle probably isn't bent.

My bet is that something wasn't tightened properly, or that a ball joint or bushing somewhere is bad and didn't get replaced as it should have. For example, my neighbor's Jeep Wrangler had to get its entire rear axle/housing replaced after an accident. He asked me to ride in it to hear a noise after he got it back, and it turned out they had loosened the front trailing arm bushing bolts to let the arms swing down for the axle replacement, but they forgot to tighten them again after it was all back together. He had brought it back to them two or three times to find and fix the noise, and they had no clue. I'm a social worker and diagnosed it in 30 seconds and fixed it in five. This shit happens all the time. In your case, I would nut-and-bolt every part of the rear suspension, and anything else they touched. Or I would have a shop do that. I bet something is loose or bad.