All Moser and Strange Engineering rear ends come with a 1350 pinion yoke that requires a 1350 u-joint for connection to the driveshaft. If you have a 1978 or later GM vehicle (G-Body, F-Body, S-10, etc.) your factory rear end has a 3R style u-joint.
The option box on rear ends for these vehicles that asks if you need a conversion u-joint provides you with a 3R to 1350 conversion u-joint. You hook the 3R end of the u-joint to your factory driveshaft and the 1350 end of the u-joint to the new rear end. You do not need to shorten your driveshaft for 12 bolt or Ford 9" rear ends. If you are getting a Dana 60 rear end then the drivehaft must be shortened 1.625". We also offer chrome moly driveshafts that will directly connect to the rear ends that we sell.
If you have a pre-1978 GM vehicle you probably have a 1310 u-joint, although this can vary and you should confirm what joint you have from the factory. For other vehicle applications you would just need to get a conversion u-joint (available at NAPA, Autozone, etc.) that goes from 1350 to whatever your driveshaft u-joint type is.