Is an 8-1/4" rearend the same for all years?

Yea, what Grinder said.

The rear end I.D also went to 8-3/8 somewhere in time, but from what I'm told, there still the same rear end.

Beside width as a difference, theres also spring mounting pads that are wider, possible brake differences in the width of drum used, possible shock mounting differences.

The newer rears that anti lock braking systems have a slightly different ring gear that holds a slight difference for the anti lock up sensor. It's kind of like a flywheel with magnets.