One-piece / single driveshaft DIAMETER

2013 4x4 double cab short bed. I have the stereotypical driveline vibe 5-10mph with a 2" AAL. I have read most of the pertinent information on one-piece driveshafts on this and other sites. What I haven't read about is what diameter one-piece driveshafts people are buying. Most look to be about 3" - 3.5".

I was talking to a reputable driveline shop here in Seattle and told him I want a one-piece with a double cardan off the tcase and a u-joint at the pinion. After measuring driveline dimensions he says, based upon my rear axle ratio (3.73) and tire size (265/70R17), that I need a steel driveshaft that is 4" in diameter. He says that a 3.5" will start to "jump rope" at ~80mph. A 4" will do the same but at ~90mph.

A couple issues with this:
1) If the driveshaft will disintegrate at ~80mph with my setup (which is a very common one) then nearly every guy that purchased a 3.5" Tom Woods driveshaft would be telling us about it by now.
2) I have not seen anyone one this site that has a single-piece driveshaft that is 4" in diameter. The one google image I found looked to be roughly 4" and it had grind marks where it touched the carrier bearing bracket.

Need some feedback from guys that have the one-piece driveshafts. What calculations did your driveline shop make about max speed vs diameter and why did they choose the size they did? Thanks for sharing your experience.

 

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I had a shop here tell me the same thing