We're told Miller's casket was loaded onto a plane in L.A. Wednesday morning and is on its way to Pittsburgh. Our sources say there will be a Jewish service for him at a chapel in the city later this week.

Mac's fans already held a public vigil for him Tuesday night at Pittsburgh's Blue Slide Park.

We broke the story ... Mac was found dead last week at his San Fernando Valley home from an apparent overdose. A tiny amount of white powder was found in the house when cops searched for clues as to how he died, but the cause is still unknown ... according to his death certificate.

The L.A. County Coroner is awaiting toxicology results before officially making a ruling ... which could be a few more weeks.