Currently there are no antirootkits for x64 (as far as average kernel mode rootkits for x64).
PatchGuard, driver signing and new operation system kernel model doing their job.
You can try something from AV companies (most their antirootkits now embedded inside products solutions) and Resplendence
AFAIR some of them were claimed as x64 compatible. However Resplendence tools are weak even on x32 so I doubt in their usefulness.
Also some user mode only antirootkits can work on x64. How well? This is up to you to test.