Large Diameter vs Small Diameter Driveshaft Question

I remember reading on here they are different lengths as well between the 8.75 and 7.25. From personal experience I know that the slant six driveshaft in my duster now is smaller than the one from my parts v8 duster. Same year, same trans, and rear just different size diameter driveshaft like you say. There's a reason they did that from the factory I'm sure.