Almost all the 2BRs at Hilton Head are dedicated. The only ones that are lockoffs are the ones in the main lodge and the ones under the Grand Villas (16 of them in all, I believe).
If you need a handicapped accessible 2BR, you will be in the main building.
If you're going during high season, when the resort is at capacity, you will have a dedicated 2BR (unless you need handicapped accessible), as the only 1BRs and studios are the ones that make up the lockoffs. During busy times, the 1BRs and studios usually fill up quickly.
When are you planning to go? Were you specifically wanting a lockoff or a dedicated?