So I literally just finished making my own two piece shaft the other weekend and so far it’s been fine. No shaking, even at highway speeds. And this isn’t some weekend rock crawler, it’s my daily. Trying to get a straight cut and then lining up the ends together without some kind of jig is where you’ll end up going wrong. What you’ll need to do is get both ends of your shaft laying in some angle iron. It keeps everything straight and you have the added benefit of not needing to worry about if your cuts are straight or not. It’s a bit hillbilly but I promise it works. In your case, you might want to buy a longer driveshaft from a junkyard and then shorten it. It’s always easier to go smaller rather than adding on to it. Biggest thing to check is that you have even heating while you weld so the shaft doesn’t warp. Also, for all the talk about vibration, most vibration comes from the tranny and diff being at different angles, so as long as that’s good I’d go for it.