Desperately trying to find out the title to this book?

Ok so back when I was in the 6th grade in the late 90s I started reading a book for our individual reading assignments during class. It was near the end of the year and I never got to finish it. For the past few years I have been searching for it with no luck so I am hoping perhaps somebody might be able to shed some light on the subject. It is a children's/young adult chapter paperback book. It had to have been published in 98 or earlier. The book is about a young girl who goes to live with a family member, either her grandmother or her aunt, I can't remember. Some how she aquires this really old doll that transports her back to the pioneer days on the wagon trail. She is not herself though, it is like she is living in an already existant girl's body who was the original owner of the doll. The original girl can go back and forth in time with the doll. The pioneer girl ends up sick with like small pox or yellow fever or something which makes the original girl sick. That is as far as I got.