The driveshaft flange, or better known as a splined yoke, has to have shims between it and the seal collar. This applies the suspension loads to the main differential bearings which are massive and can easily handle the load. If there are no shims, or not enough, the suspension loads are applied to the output shaft and to the diff's spider gears. They are not strong enough and will quickly fail. Bits and pieces can then circulate through the transaxle causing further damage.
The roll pin(s) simply holds the assembly together and does not take any loads.
PLEASE MAKE SURE YOUR OUTPUT YOKES ARE PROPERLY SHIMMED!