To expand a bit on what Limblips said, there is a conventional bushing in the housing that the lower end of the shaft has to go into. There is also a thrust washer that goes on the shaft next to the gear. The thrust washer usually gets out of place. The easiest way is usually to fish the thrust washer out of the housing and put it on the shaft. Use some grease to stick the thrust washer onto the shaft.
After you get it in place, install the retainer and upper bushing. Check for end play of the shaft (by turning the shaft through its free play). If there is any, check the gasket on the retainer. If the gasket is too thick, it will add end play that can be reduced with a thinner gasket. If you have end play that can't be corrected with a gasket, other steps can be taken.
