A driveshaft assembly for a downhole motor comprises a housing and a driveshaft rotatably disposed within the housing. The driveshaft has a central axis and a first end. Further, the driveshaft assembly comprises a driveshaft adapter having a first end coupled to the first end of the driveshaft with a universal joint. The first end of the driveshaft adapter includes a counterbore. The first end of the driveshaft and the universal joint are disposed in the counterbore. Still further, the driveshaft assembly comprises an end cap disposed about the driveshaft and coaxially coupled to the adapter. Moreover, the driveshaft assembly comprises a seal boot disposed about the driveshaft and adapted to restrict fluid flow through a annular gap positioned between the driveshaft and the end cap. The driveshaft assembly also comprises a locking assembly adapted to restrict the end cap from moving axially relative to the adapter.