Another '74 Overlander checking in...
I contemplated pulling the skin and checking things out back there this past winter when I replaced the axles. In the end, I opted not to because I don't have the serious signs of separation. There's about a 1/4" gap on one side, but to me that wasn't enough to warrant ripping apart the entire rear end when none of the other classic separation signs were present.
Keep in mind that such a thing is not uncommon and the repairs required are not specific to just Overlanders. The longer trailers (Ambassadors, Sovereigns) were even more so susceptible when outfitted with the rear bathroom. There's numerous threads here about having / diagnosing / fixing rear end separation. Don't limit yourself to discussions only about Overlanders for your repairs.