C.J.
Probably not the answer you're looking for, but is there a reason you can't let her nurse for a nap? I've let all mine (I have six) wean themselves when they were ready. As it is, my 4 year old still wants to nurse for naptimes and when she's sick/hurt. Of course, I'm also nursing my 18 month old.
Naps are extremely important for young children (even my 7 y/o still takes the occasional nap). If nursing is the only way to get her down right now, let her nurse. Tell her she can only nurse for 5-10 minutes, and then stick to that. Slowly cut that down by 30 seconds at a time until she isn't nursing anymore, but is just getting "cuddle time".