I once made the mistake of trying to set up a tiller to be used on two different tractors. One needed the PTO to be about 2 inches shorter than the other. It worked OK - for a while. But eventually, while using the tiller on the tractor that should have had the longer shaft, the tube split wide open where the pieces slide together. It never slid apart, which is what I was watching for, just split because there was too little of the two parts driving each other.
I fixed it using Mud2money's method. Bought the tube on Amazon as I recall, but it's been long enough that I'm not sure of that now. I can only use the tiller on one of my tractors, but that turns out to be a small price to pay.