I was in kind of the same circumstance. All of the professors I am familiar with were in Geology which is not considered a science and DMU specifically required LORS from basic science professors. I only came back and took ~3 pre-med courses and the rest were all done about 10 years ago. I ended up getting LORs from both my Genetics professor and the Organic II lab head. Both of them went back and discussed with my TAs and actually quoted, from what I was told, the TAs in the LORs. Basically all my professors could say is that I got an A so they needed the extra help. However, each professor kind of interviewed me for about an hour also in order to feel comfortable writing the letter. I think I am pretty good at putting on a puppy dog face and I think it helped a little so I would recommend that too.