When i put my car together(many moons ago) The driveshaft guy asked me to put my car on a drive on hoist and measure from the trans output shaft tip to the center of the ujoint saddle on the diff. I gave him that measurement and the slip yoke and he built me a shaft. It's been in there 25+ yrs with zero issues. Could your vibration be caused by pinion angle and not the shaft? I changed ride height at one point and developed a vibration on light accel..... a shim under the leafs cured it.