Originally Posted by SquidPuppet

Cool. Maybe the rear hub is all good now.

Open that bottom bracket up and see what's going on in there. Ashtabula (one piece) cranks are stupidly simple to service and parts are dirt cheap. Just FYI, the lock nut and cone on the non-drive side are reverse threaded. Pack the snot out of it. Also, you will never achieve a perfect bearing preload adjustment on an Ashtabula crank, never, no matter what, so don't kill yourself trying. It will either have a tiny bit of play, or be play-free and feel a tiny bit lumpy. Personally, I go for lumpy.

A thought...

I'm just sharing this because you mentioned that you were more familiar with road and MTBs. With derailleurs I assume. And derailleurs keep the chain properly tensioned for you. Your cruiser chain is different, obviously. Make sure the chain isn't too tight. If it is you will definitely feel and hear a binding that could be confused with other issues. It's bad too, for everything in the drive train. The proper tension for a single speed chain is NO tension. You want slack. Visible slack. Don't be afraid to run it a little sloppy. Obviously you don't want to be able to derail it, but the zone between too tight and derail is LARGE.