Far as I know, the issue is more driveshaft
length, than driveshaft
load.
The driveshaft resonates/flexes at a frequency dependent on length, material, wall thickness, and diameter. Short driveshaft flex frequency is higher than long driveshafts, steel is different than aluminum or carbon-fiber, and a large diameter flexes less than a small diameter.
Calculate Driveshaft Critical Speed - by Wallace Racing Home of Pontiac Powered Firebirds,Trans Ams,Pontiac Powered Dragsters,Pontiac Power Rules!
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The idea is that the driveshaft flex "critical speed" has to be higher than the vehicle will ever spin that driveshaft. So instead of one long and whippy shaft, they go with two shorter shafts. Look under an old front-engine school bus sometime--they've got more driveshaft segments and carrier bearings than you can shake a stick at.
This is also why longer vehicles generally end-up with longer transmission housings/tailshafts--to shorten the driveshaft length to something manageable. Cadillac TH400 transmissions had the longest tailshafts of all, full-size cars got the next-longest, and compact/intermediate cars got the short tailshafts.
Although the stresses are different, the things that make a good driveshaft
also make a good pushrod--material, wall-thickness, diameter, length, etc.