Broken prop shaft!

To check for straightness you would set your shaft in 2 V blocks. Set up a dial indicator and you can slide the dial indicator to any spot on your shaft and while you rotate the shaft check for straightness.   Not hard to do and no special surface needed.   You can get a stand mount dial indicator for not too much money or borrow one.
I keep one on hand, it comes in handy often.

This happened to me a couple months ago. Take prop shaft out and find a flat surface (kitchen table, ping pong, etc.) and roll it across. If you see the shaft rolling with a warp it is bent and replace it.

New shaft installed and all seems ok!! I'm 90% sure the problem was both bad alignment AND seal was too tight.. my wax but had to be very loose in order for any water to cool it.. And by far the best way to align, is by bolting a shim in the tight side first,- that way you can observe both the + and - while rotating the coupler. .. I found out my engine was out of alignment from right to left, not just up and down. - when I rebuilt the stringers and floor, it three everything way off. Thanks for everyone's help... Now I have to figure out why it shifts so hard into reverse...