need rear drive shaft lenght 4x4 5speed sclb dually

I need rear drive shaft length 4x4 5speed sclb dually. im going to do a 80 swap on my truck and ive been told ill need to shorten my drive shaft, but my buddy that has a srw 80 under his truck has the same size drive shaft that's in my truck that only has a 70. (i converted mine to 5 speed)