Originally Posted by spasticrobot
edit: This is definitely the problem. I just put the cassette back on the original wheel and it's sitting tight. I guess I need to put a spacer on the new wheel. Can anyone tell me if this will mess up my rear derailleur's indexing?
You didn't say what vintage of Ultegra you are using; if you are still using 7 speed, the cassette is not as deep as 8/9/10.
Put the spacer on before installing the cassette; with a little luck, you won't have to adjust. That said, if you don't know how to adjust a rear derailleur it's time to learn
Good luck.
Ed