Interesting . . . do you happen to have a pic of your weld job??
On a JD front mower idler pulley I had no wobble from pulley to its shaft but wobble of the shaft relative to the plane of the belt owing to the "buckling" or "oil-canning" of the stamped, sheet metal deck itself. The heavier load of tall, thick grass may be causing your deck to buckle. I welded some stiffening flat stock to the underside of the deck, and that solved the problem.