Are you applying to the UIC MSTP(chicago) program or the UIUC medical Scholars program(urbana)? The UIC application says: "The College of Medicine letters of recommendation, depending on their content, may need to be supplemented by separate letters submitted to our program office. One way or the other, our program requires at least one letter of recommendation from faculty or superiors familiar with your research. These letters should be mailed directly to the MD/PhD Training Program office (address in item 2 above)." If you are applying to the UIUC program, the letters are due on December 31st I believe, so you still have time to get some. If you don't just list the names of the people who have written you letters.
I am applying to both, I am having problem with the Medical Scholars Program. I have only one rec letter that talks about my research, and its in the application packet that I sent to the school and the program as well. I think they want three though...when I was filling out the online secondary I couldn't get away with only one reference.
I don't think that all of the letters have to talk about your research. If discuss your ability for combined study then you should be ok. That's what I did. I only have one letter specifically about my research from my PI. For the other letters I just told my profs that I'm applying MD/PhD and the letter should reflect that. If you don't have any letters like that then you might want to try and get some before the deadline.