I skimmed this thread and will tell you that I chased driveshaft vibrations in my Duster last year because I guessed the right length based on “factory spec length” and what I measured while car was on a 2 post lift and was WRONG. Wrong cost me several weeks of enjoyment and $800 because I decided to call an expert (Strange Engineering) and they told me how to measure it. They built a new driveshaft for me and it was 3/8” longer than what I took out (cap to cap) and my violent driveshaft vibration was gone.
You need to measure with the car’s weight on the suspension and never forget facts like +2” leaf springs and pinion angle shims to fix pinion angle will affect your driveshaft length. Even if the car is built to factory spec it does not mean it takes a factory length driveshaft, mine is 1/4” longer than posted factory spec due to “other variables.” Measure every time…
Strange Engineering treated me well, I knew it would be expensive, but I knew they are among the best in custom length Mopar driveshafts. And being mid summer I wanted to obtain the correct length driveshaft and soonest possible, and Strange Engineering hooked me up. Shortest lead time and knowledgeable techs you can actually talk to on the phone.
You can go where you want but know that too long risks broken transmission and too short is not fun vibrations (scary) on the highway with 3.91 gears at even 60 mph. So measure correctly and buy once, I measured wrong and bought twice.