Maybe it's cause I have only worked on a 8" diff, but I have never heard of a cross shaft. Do you mean pinion shaft? Anyways, you shouldn't because the only real different in gears is the the ratio of pinion teeth to ring teeth.
Yea its the pinion shaft, RPM outlet called it a modified cross shaft and said you only needed it with 4:10s or higher, anyways thanks, I think they were trying to make an extra buck.