Just to clarify (I also go to UCD) most letters are included in the PHPAC file unless they come from an inappropriate source or for some reason they feel a letter obviously does not represent you. They do not choose the best ones to send and leave the others behind. And, if you only have 2 letters from science profs, tough, there going to send them even if they suck because they will include a minimum of 3 letters, 2 being from science profs.
To the OP, I assume you must have taken the CHE 128/129 series rather than 118? Yes, it would be fine to get a letter from your lab instructor. As other people mentioned you could have the lecture prof co-sign if you wanted. An approach that I used was asking them if they would be willing to let me take them out to lunch (on the Dean Whitter Fund - learn what it is if you don't know!) to get to know you better and they can decide if they would be able to write a LOR for med school on my behalf (OK, don't phrase it exactly that way, but basically the point is to MEET with them after giving them your PS, transcript, resume/CV, etc. Tell them about your future goals and a little about your life, that way they might be able to write a little more than "he got an A.") Some profs are much more willing to do this than others, so ask around (if you want to PM me about certain profs you've had I can do my best to help.) I felt the whole take out to lunch thing was sucking up, but the Dean whitter fund totally saves you on this one because that is it's purpose so you can totally brush it off on that! (purpose = to encourage relations between students and faculty.)