Lima clutch and joint in Lima, Ohio built the custom front shaft. They actually had what I needed in stock, so I assume I wasn't the first person to come in with the issue. They cut it to size, welded, and balanced it while I waited. The counter guy was pretty smart and I recommend them if you're needing work.
I was told, that for any driveshaft to work, the joint angles on each side need to match within a half of a degree, and be in opposite directions. So for a single rear driveshaft to "work", most vehicles have the transfer case/transmission angled down, requiring the pinion to be at an up angle the same amount. You can also run into leaf spring flex under accel/decel, allowing the rear joint angle to change, which needs to be considered too. I mention that because that sierra didn't make noise under power, only decel. Pointing the pinion angle up higher, lowered the angle difference when the truck was under decel, which eliminated the noise. Truthfully, installing a set of traction bars or anti-wrap bars probably would have worked on the rear of that truck too.
Front shafts are a different scenario, as the transfer angle is now pointed up, requiring the front pinion to be pointed down in order to cancel the difference out. I couldn't do that on the sierra without the joint angles being out to lunch, so installing a double cardan joint on the rear of the shaft eliminated the need to have the front joint # match the rear joint #. Then, all I had to do was get the pinion-driveshaft angle as near to zero as I could.
Spicer has a calculator and smart phone app you can use
https://spicerparts.com/calculators/driveline-operating-angle-calculator#more
And here's a video on driveline angles, velocities, and the issues they present
xc_hide_links_from_guests_guests_error_hide_media