I have a 20 month old that has ALWAYS been tough to get down for a nap. Just recently, and I mean only in the past week, I started putting her in her crib with her baby dolls to play. I tell her that it is time to have the babies go night-night and give her a blanket and bottle for them. Then, I'll leave the room, but with the door open and put some mellow music on in the living room. I try to put the music on as loud as possible without it being blaring. She will get bored and call (and whine/cry a little bit) for a while, but eventually she will fall asleep.
This is a kid that would cry and scream until she threw up if I walked out of the room and left her in the crib to go to sleep at night. I don't tell her that it is nap time or that I am leaving the room for good. She can hear me in the kitchen and doing other things, so I don't think she feels like she is being left. My daughter also is about at the height limit for the crib, but I am considering getting one of those nets that goes on top. I can't trust her in her room by herself yet.
One more thought is to be sure the cord on your blinds or anything else in the room is taken care of. A friend of a friend had her three year old daughter die after getting strangled in the cord blinds. Sorry to have to mention it, but it is something I am concerned about when we do move her from the crib.
Good luck, you have a tough situation on your hands!