Driveshaft questions on T-5 swap

[Turborg]

While inquiring about driveshafts at local shops I was told by two out of three that a drive shaft that is 60 inches long (approx six inches shorter than what is required on the t-5 swap) and 3" in diameter will reach its critical speed at 4000 RPM. Critical speed is defined as the rpm at which the driveshaft can fail if speed exceeded. I also checked on the internet under the heading "driveshaft critical speed" and saw the charts the driveshaft shops were using. My Volvo would be going less than 100 mph at this driveshaft speed. I would appreciate any input on this as I am ready for a driveshaft now.
I am leaning toward having the short shaft on my shortened and beefing up the carrier bearing.