Ok, as indicated below I own a 2011 F-250, FX-4, 156" WB... I came across this TSB which addresses F-250 Rear Sag. The TSB directs the replacement of the drive shaft. I stopped by my local Ford dealer and verified that an F-350, 4x4, 156" WB, 6.7L uses the same drive shaft. The TSB replaces the 2" blocks on the rear axle with the 4" blocks that come on the F-350. If the same drive shaft is used, why would it be required to replace the drive shaft for the performance of this TSB?