There isn't a separate "correction" message.  If you don't have lat/lon, etc, then it is because the software reading that message is looking at one value in that message that indicates the type of GPS, and because it doesn't recognize the value of 5 or whatever is used to indicate a gps location corrected with XP, it thinks that there is no GPS, so it doesn't forward the message.   The correction has already been applied in the GPS receiver.   That's what is so frustrating about the situation.

Short of Deere fixing their gps reader function, it would be interesting to see if there were some inexpensive little box that could consume a GPS stream on one serial port and forward the messages on a different port, but in the process scan the message that has the GPS type value, and change it back to just plain "differential".    There are probably people out here who know of programmable boxes w/o a UI.   I would consider writing the code if someone knew of a hardware platform to do it.