I have been doing a lot of last-minute transcon travel towards the end of this year, and it has been nearly impossible to even purchase a J seat on most EWR-LAX/SFO flights any less than a week out. Dunno about the economy loads, nor whether this is just a seasonal thing, but so far there isn't any evidence that UA is having trouble filling the J cabin of their EWR-LAXSFO flights.