84 regal rear ends

I would say no, not all regals had posi.
When I ordered my GN in '86 posi was an option. It was standard in '87, but my guess is only on the GN's. I say this as there are a lot of guys that have '87 T-types and Turbo T's that run one air bag to compensate for the lack of posi.
The posi cars had a tag on the diff cover that said to use posi fluid, but the tags may be long gone due to slackers not putting it back on after service.
it was an extra cost option on all regals for all years except for the extended run of 87GNs built after sept 1987 when it became standard