I dunno about the dead coming back (since nobody ever dies in Power Rangers except monsters), but I've always kind of figured that when depowered Rangers come back for reunions, they're temporarily able to tap into the same power source that the current Ranger team is using. It was established a few years back, I believe in Operation Overdrive and Jungle Fury, that all Ranger teams ultimately derive their powers from the "Universal Morphing Grid." So any old Ranger technology/magic might still be able to piggyback onto the UMG link that powers any currently active Ranger tech/magic.
Which was how Jason, Tommy, and Adam were able to return in their prior MMPR and Zeo incarnations for the reunion shows.
Of course, the writers could make up the rules as they go along.