I think this might help a lot. I am looking through a J1850 book and it shows a few examples. They use a HIP7010 or HIP7030A2 as the handler. This should take care of the protocol aspect. Hook it into a HIP7020 Transceiver and you should be good to go. I didn't study the data sheet too much, but I think this will simplify things. Here is a link to the data sheet for one of the handlers.

http://www.intersil.com/data/fn/fn3644.pdf