Rear Derailleur replacement

Here's the deal. I've spent the better part of the last 3 nights trying (operative word is trying) to get my shifting silky. I have new cables and housings to put on and I have a new rear XT to put on, BUT, I didn't want to do any of this until the winter was over. I was hoping to just stretch the old LX to say mid March. Unfortunately I fear I have no choice except to do replacement unless any of you can give me some good advice.