Craftsman mower scalps lawn- need repair advice

My guess is that one mandrel assembly is out of alignment on my one year old craftsman 20hp 42" model 917.27682.
It was scalping the lawn when going over bumps at low heights. I thought it was a bent blade but did the same with a new blade.
I removed the deck and did some tests. One blade is at the same depth when rotated 360 (so works fine) while the other is low on one blade tip and high on the other. It is slightly beyond the edge of the bottom of the deck, so obviously will scalp the lawn if the deck bottoms out.
If the blade is removed and replaced about 180, the low/high reverses.
If the blades are swapped, the low/high is still about the same on the same mandrel.
I noticed that there are four bolts around the pulley/blade drive assembly and wonder if the alignment can be corrected?
I know I can probably shim it at the blade to get it into alignment, but is there another fix?
Or is it beyond repair? I am reluctant to replace the pulley assembly in case the deck itself has an alignment problem.
Unfortunately, the machine did not come with a IPB of the mower deck, only the tractor, so I cannot figure out just what is involved in the pulley/blade drive assembly.
Any help appreciated.
GA
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I wouldn't try to shim it at the blade - but you might try 90* instead of 180 for a temporary fix. Does the belt look like it is riding OK in the pulley ? Shimming at the "4 bolts" could knock the belt-pulley out of line enough to wear belts out but that is the only place I would shim. Another possibility is to file down the high spot on the shaft if that is the problem.
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Srgnt Billko wrote:

below the bottom of the deck.
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