self larnin' goin' on--narrowing rear axle

If you attempt to use tubing you would need to put it on V blocks and use a dial indicator to check if it's true (even on a normal alignment bar it's good to check this). You want it to be true to under ".004. I'm not sure tubing will give you that accuracy though, the slightest bit of flex in the tubing could spell disaster as well. There's a reason those alignment bars cost $100+.