I measured the circumference of both driveshafts (11") and divided the numbers by pi (3.1415926) to get the diameter.
Well I just crawled under mine. The mid shaft circumference is 11 5/16" using a fabric tape. So, 11.3125"/3.1415926 = 3.6008806" diameter. I no longer have my OEM but I vividly recall it being significantly smaller.