As this surname was present in many different places in XX century, it would help if you could post any other information you have on them - like  dates of immigration, ship manifests or naturalization /immigration documents.

Do you have any infromation on their place of origin?

He was born in Majdan, Gornji Milanovac, Serbia.  He immigrated to U.S/Canada in 1950/51. 

I have an ancestry document that says a Bogoljub Markovic left Bremerhaven for New York on July 7, 1950.  Everything seems pretty accurate as far as birth place (Yugoslavia), age and approx. birth. is concerned. It was on the ship General J H McRae. 

However, family members feel that he came over on the M. S. Nellie on December 13, 1951.

He did go back and forth to visit a daughter that had Polio and eventually died from the disease.  I think maybe that was where they are may be getting the M.S Nellie ship/trip from?  I'm not certain though.

He was married and had 3 children (between the years of 1945-1951) in Kufstein, Austria before immigrating.

You need to contact the municipal office in Gornji Milanovac where the vital records are stored. For people born in XX century data is available in civil vital records but there are limitations to what info and how they will provide because of privacy laws in Serbia. Also all requests must be submitted in official format and in Serbian language.

Once you know the names of parents  it will be easier to track down living relatives in Serbia.

For church vital records you need to contact Serbian Orthodox Church, as church vital records are in their posession.

For assistance and field research I recommend our team member Petar Demić who lives in that area and has worked extensively in the archives. He is an university student majoring in history and a very thorough genealogist.

his username and profile:

http://www.rodoslovlje.com/user/okir1793

If you share photos of the people you are researching I will repost them on "Rodoslovlje" facebook page  on your behalf with a request for info on relatives in both Serbian and English language.

"he was married and had 3 children (between the years 1945-1951) in Kufstein, Austria before immigrating. "

guess it was the DP Camp there - pictures to find at this website of an Ukrainian: http://www.kogevins.com/english/eugene-and-raissa-kogevin/dp-camp-in-kufstein-austria/