Freelancing Advice

I could use some advice. I am currently employed at a design agency (and have been for the past two years) where my primary role is a web production developer/php programmer. Most of my duties consist of developing back-end functionality, or css/xhtml buildouts of site concepts by another designer. While I do have an educational background in design, my work experience has tended to shift in this direction, based on necessity on the part of the agency (too many designers, not enough web-dudes).

While I very much enjoy the challenges of making another person's design work in a consistent manner, and the problem solving that comes along with web programming, I am becoming increasingly worried about being pigeonholed into a purely production role and am looking to start freelancing on the side.

My problem is this: since I have spent the past two years making mostly small decisions based on other people's concepts, I have little portfolio work to show in which I can claim 100% creative involvement, and it seems wrong to me to show someone else's concept on my own portfolio site. I could post work from college, but some of that is 4-5 years old at this point and, quite frankly, kind of embarrassing. The only other option I can think of is to produce mock ups of my capabilities, in terms of programming and design, which also seems less than ideal, since that would do nothing to demonstrate my ability to work with client demands.

Sorry for being so long winded, I am basically just hoping to get some ideas or suggestions about what kind of work I can put in to get myself into a more satisfying place. Has anyone else encountered this problem? If so, what steps did you take to get out?