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.