It depends on which, and where. It's often hard to isolate, or maybe more than one is out of balance.
We have a minivan that had a vibration years ago (due to a rogue curb the wife tried to fight with, if you catch my drift..) It turned out that both the front and back passenger-side tires were WAY out of balance. Thus, rotating front to back would have just swapped two tires that were out of balance and the vibration would have persisted the same way.
I get what you're saying and I won't beat this dead horse after this time, and you may indeed have the power steering rack issue, but I really would suggest getting your tires balanced before going farther on the vibration issue. It's along the lines of checking tranny fluid first if you have any shifting issues, or even checking fuel level and your battery if you have starting issues. Lots of people overlook that and replace things unnecessarily only to eventually find out that a simple tire rebalance was all that was needed. If they're only a little out of balance, you could only notice it when you hold the steering wheel, as you said. And, if it's only at higher speeds, that's again a clue to a balance issue.
Anyway.... Good luck, and please let us know the outcome!