Okay, first we role model how our kids are to behave. Do you get excited when you go and see them? Do you give out hugs and kisses when greeting? This would be the first thing I would start doing if in fact that is what you expect from her.
Secondly, it appears there isn't a strong bond there, so I would suggest more frequent visits even if not very long. This makes the greeting with them not such a special thing and then just a "hi and a smile" may suffice and no uncomfortableness for anyone. If that doesn't help or isn't an option, then try leaving her with them alone for short periods of time so that they can bond without you there. If you don't feel comfortable with them watching her without you, then perhaps she shouldn't be hugging and kissing on them. If you trust, then she will too.
Grandparents have every right to feel hurt. Your daughter has every right to feel the way she does. You are doing great by seeking out solutions so that the people you love aren't hurt. So I say Kuddos to you and I agree that any form of discipline, manipulation, or treats is wrong.
I had this same problem with both my kids. My inlaws are in the same town as us and kids saw them about 3 times a week and spent every Sunday afternoon alone with them. They get excited when they are around and give hugs & kisses as freely as with us. My parents live about 30 minutes away and kids see them about 2-4 hours every month. So, when we got there, they acted shy and nervous for about an hour then they would loosen up. I now take them to see them at least once a week even if it is just for 30 minutes. They seem to remember them and now give out hugs often to them as well. They do favor my dad over my mom, but it is because my dad is goofy and rolls on the floor with them all the time. So, I know how uncomfortable it can be. Hope this helps and good luck to you and your little one.