Hey Craig,
 
I have nursed my stripped wiper shafts along for a few years now. I cut a small (1/4") section of fine surgical tubing and placed it around the shaft where the splines are supposed to be. It compresses in the conical space between the shaft and the wiper arm and holds pretty well. I get occasional, marginal slippage but it has lasted now three seasons and I live in Seattle and use my Vanagon as a daily driver. Of couse my threads are still intact... If yours are somewhat there but give out at the bottom of the tightening range this might still work as it lifts the arm out a bit.
 
Best of luck...

Tom Humphreys
Secondhand Sunday

www.secondhandsunday.com