FYI. I put in a TAC case for this and the answer is that it is not supported on the 3845 platform.
I found that really strange. Here was a response from them:
Andrew,
It does not look like IOS SLB is supported for that platform. Here are the supported platforms
2691 is NOT supported at all.
In 12.3 and 12.3T, 3640/3660, 3725/3745, 7200.
In 12.2S, 7200/7300/7400, native C6K.
No plans for new platforms at this time.