Nick,
Why would BOTH ends need to be swapped out? Assuming the rear to rear replacement was done, and the original 4.10 gears were used in both. There is no need to replace the front end too. Also one can as noted, rebuild the rear, unless the actual housing is broken etc too.
Now if he switched to a different ratio in the rear, then yes, you need to swap out the front. My son is in the process of doing this with his 05 K1500, going from an 8.5" rear diff with 3.42's to 4.10's. He's getting a 14bolt 9.5" RA with 4.10s, along with the front from the same truck.
Marty