It's been quite a while, but what I did, iirc, was insert the yoke as far as it would go into the transmission (while still attached to the drive shaft...) and measured what it would take to be able to fit the caps into the rear axle yoke. Once that measurement is taken you can tell the shop how much you need it shortened. They're gonna go from the 2 ends to measure the shaft anyway, so it doesn't necessarily have to be assembled when it goes to them, but they'll want it so they can rebalance the assembly. They should throw new U joints in it as well, but I suppose that depends on the shop and how they roll.