What do I if I have not gotten a decision on my I-751 when it's coming up on a year and am living overseas on government orders?

You are still a lawful resident despite l the 751 extension expiration. I would recommend first that you contact the American Service section of the US consulate with jurisdiction over your foreign residence, explain your situation. There is a vehicle known as an SB visa for returning residents, lawful resident, inquire as to their willingness to issue one for you when you are ready to return.

You should contact U.S Embassy for guidance which ever country you are posted in

You may be able to get a "transportation letter" (sometimes referred to as a "boarding foil") from the US consulate where you are, allowing return to the US. Would necessarily be able to use this as evidence of conditional permanent residence status afte ryour return, but you could then make an Infopass appointment with a local USCIS office to obtain a temporary green card stamp as evidence of ongoing status and to allow further travel (as long as there still has ben no decision on the I-751. Not clear form your message whether you have obtained a Reentry permit to preserve your permanent residence status given how long you you may be overseas, but this too is a travel document. You need to speak with an attorney about preserving your permanent residence status and about the documentation involved here.

You are still a permanent resident despite the fact that your card expiration date and the impending expiration of your Extension Letter. If you have an opportunity to visit a local USCIS office on a return trip home, you may be able to get another extension. If you are unable to return, it's best to contact an attorney to discuss your options.