I've never had a driveshaft shortened but having Dodge trucks with driveshaft splines I've had more than one spline replaced. Always had them balanced. One time the shop screwed up the phasing and I got a fairly bad vibration from it. Took it back to the shop, they cut it open again, rewelded it, and life was good. No balance the second time through. My conclusion? Phasing is more important than balance.