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You can buy just the shaft from some of the vendors on the site. It might take a little searching of the catalogs and work to replace them. Have you tried to tighten the screws that hold the handles on? I know a dumb question but sometimes it happens.
If your handy with a welder, highspeed cut off wheel, and files? You can also add more metal to the splines. Just take your time and it will come out fine.
I know Mac's has the window regulator shafts for $20.00 per and inner door shaft repair kits for $20.00 per as well. Call them to find out if they interchange with the 61-66 though, they are listed for the 57-60.