There have been numerous discussions on this forum about your question... and recently again, I might add. If you do a search, then you should find most of the related threads.
Here is the most recent discussion: http://forum.bacrallies.com/m-1374847063/s-12/highlight-exhaust+brake/#num12
I found a truck shop in Pasco, Washington that will reprogram the transmission software to allow the Jake Brake to operate in the higher gears and, importantly, not force a gear reduction from 6th to 4th or 5th to 4th. My brother had his reprogrammed last week, and I plan to have mine reprogrammed this week. Cost is one hour minimum shop time... or about $100, more or less depending upon the shop's rate.