My shifter has begun to rattle during acceleration. I understand that the shifter needs to be repacked with grease. How do you remove the plastic ring that holds the rubber boot in place? What grease is best for the shifter?

 

Is it rattling or just moving around a bit? Movement is normal. Anyway, check the following even though it may not be the same as what you've got. the procedure is the same:
https://www.s2ki.com/forums/index.php?showtopic=41390

ps. Check the FAQs in the "Technical" section of UTH.