68451 or a custom SUN-like (SRAM, kind of like a PDP11) MMU, there was a guy who went around Silicon Valley in the mid 80s designing SUN-like MMUs for companies, they were all different, and some were broken (couldn't protect user space from kernel space).

68000s however had a problem: they couldn't return correctly from a page (MMU) fault (68010s fixed that) for a pre-VM (pre BSD or SVR2) UNIX world - however you could get around this with a few smarts