All my printouts on my Phaser 8400 is fuzzy and

What normally causes that is the Drum motor encoder picks up the lubrucant from the drum and makes the drum spin to fast and or just out of sync.
It is located on the right side of the machine. I reference that looking from the front. To remove the right cover you remove one torx screw from the top back and one inside the paper path door in front, near the top. Then when those 2 are removed you lift up and out on the cover.
The encoder is on the end of the drum shaft. You can remove it and clean it by pouring alcohol over it, wiping it gently then allow it time to dry.
That should take care of it. If not then the encoder will need replaced.
If you go to Xerox.com and click on support and drivers, enter 8400 in the search box, and it may be on the support page or the documentation page, but there is a parts list on the site that you can look at as a reference or also to get a part number to order a new one if needed. Using the parts list is the best way to be certain you are getting to the correct part and not guessing.
But I am about 90 percent certain that the encoder is the issue.