Different sized u-joints?

I am replacing my u-joints on my truck for the second time or really rather just one u joint, the one residing In the middle of the driveshaft. The time i replaced this one the guy at the auto parts store informed me there were two differnt sizes on my driveshaft the u joint coming off the transfer case and the u joint at the diff are the same size but the one in the middle had a smaller diameter? Is this normal and does anyone know what u joint I need to get for it. I called the parts store and all they recommended to me was to bring it in and compare it but this is my only vehicle so that makes it a but difficult to do so.

 

The guy at the autoparts store is retarded. All three universal joints on the rear drive shaft are the same size.

 

Front driveshaft has a smaller u joint size than rear driveshaft. But the 3 rear are the same size and the 2 front are the same size with respect to eachother.