Think about it this way - your daughter has come to completely depend upon you to go to sleep at night. After all, that's been the 'routine' for the past 18 months. In my opinion, it doesn't matter what you do (cry it out or gentler cry it out) you're going to face resistance and she isn't going to like the change one bit at all.
Since she can nap fine, that indicates to me that she is capable of putting herself to sleep on her own. Since you have decided that it is time for her to follow that same practice at night, then you need to make your decision and stick with it. Personally, if I were in this situation, I'd use the 'extinction' method and have her cry it out (that's just me and I know it doesn't work for everyone). Yes, she may scream for up to several hours. Yes, this might take a week or longer. Eventually she will get the idea that you're not coming back until morning, which is NOT an unreasonable thing to expect from an 18 month old. Many people think this is cruel and terrible, but again, in my opinion, sleep is just as important as nutrition and nurturing. Since she can do it for nap time, she should be able to do it at night. If you choose this method, do not go in and check on her because it only reinforces her thought of "screaming makes mommy come back".
You can also try a gradual extinction method that many parents have found to be successful. Put her in the crib, kiss her good night, then leave the room. Check back every 5 minutes, reassuring her that you are there for her but it is time for bed. The next night, stretch the intervals to 15 minutes...and gradually lengthening the intervals each night as needed. You can also do a 'chair method' where you sit next to the crib and slowly move the chair farther and farther away.
Another thing to examine might be her bedtime. Are you putting her to bed too early or too late? Ideally you want to catch her right when she is exhibiting signs of tiredness and put her to bed then (rubbing eyes, tugging ear, yawning, hands/feet warm). One mistake I made earlier with my baby is keeping him awake til daddy got home from work (he works 10-14 hour days). I'd keep him awake for daddy to see him, but by then he was so tired and cranky that trying to put him to bed was a chore. Now, as much as I don't like it, the baby goes to bed at his bedtime whether or not daddy is home. I feel badly, but I know my baby (8 months) won't always go to bed at 6:30 and it'll eventually be later.