[ChevLoRay]

Take your measurement from the centerline of the rear yoke, as in the center of the u-joint bearing cap, to the center of the splined yoke that is exiting from the tailshaft of the transmission. That measurement is the length you need. Make sure the truck is resting on the ground in its static position, not on a rack with the wheels suspended by the springs. A drive-on rack is best for this, as it offers you the best range of movement to take your readings. I'd measure mine, but it's at the body shop and it just ain't possible.

Expect to spend about $150+, in my area for a NEW fabricated one-piece driveshaft. You may be able to find one in a junkyard, but you may have to have the ends swapped-out to get it to work for you. Don't forget that it needs to be balanced.