I'm just going out on a limb here...but I'm going to guess setup was improper when the new gears and locker were installed. Possibly a little to tight like most people set them up. When gears are moving they are constantly trying to pull themselves apart, it's the nature of the beast, I set them the pinion pretty tight to the ring gear but leave 10-12 thou backlash in it for this reason. Replace everything in the diff just to cover your ass, if the client says no, make sure that warranty is void and customer signs it. G2 will be fine unless this guy works the hell out of this truck.
As you found out the L is for long, if you get into doing to many rear ends there can also be wide and thin ring gears depending on the case. A+ for doing your homework on the part numbers.
Also as mentioned if this is a 10.5 axle with 10.25 gears could be another reason for failure if the wrong kit was put in.