A833OD with worn output spline

I am using an A833OD with an iron main case behind a 440 in a street rod pickup. Having noticed a vibration when "floating" the drive train at over 70 mph. I have narrowed it down to a sloppy fit between the slip yoke and the output spline. With the main shaft in the vise with full spline contact, I am measuring more than .027 vertical end play on both a used slip yoke, and new one from Spicer. I have managed to get my hands on another shaft, and it is just as bad if not worse. In comparison, I measured a Saginaw 4-speed, and only saw.004 vertical end play. Is it normal to see the A833 this sloppy? Is there a possibility of getting the spline or slip yoke coated with a product that will tighten it up, maybe similar to the coatings on piston skirts? With 4:33 gears, the driveshaft is spinning 3300 plus at highway speed. No issues well past 100MPH as long as I either stay on the gas, or completely off it. Anywhere near neutralizing it will shake the mirror off the cab. Any guidance is much appreciated.