1. Verify that the joint shaft is not twisted or cracked.
2. Turn the joint shaft by hand and verify that the bearing rotates smoothly and freely.

Drive shaft
Joint shaft removal/installationMazda 6 Service Manual: Locking, Unlocking with Auxiliary Key
Turn the auxiliary key toward the front to lock, toward the back to unlock.
To lock
All doors lock automatically when the driver's door is locked using the auxiliary
key.
To unlock
The driver’s door unlocks when the auxiliary key is turned briefly to the unlock
position and then immediatel ...
Mazda 6 Service Manual: Rear oil seal
REAR OIL SEAL REPLACEMENT
1. Remove the flywheel. (MTX) (See CLUTCH UNIT REMOVAL/INSTALLATION)
2. Remove the drive plate. (ATX) (See DRIVE PLATE REMOVAL/INSTALLATION)
3. Remove in the order indicated in the table.
4. Install in the reverse order of removal.
Rear Oil Seal Installation Note
...