79 and older use different bearing retainers, and can be narrower.Where you at? I've got a 9" out of a bronco sitting in the shop right now. It showed up as trade and I've got it for sale on DR for $275
And yes as Paul said the ones out of an econoline are wider. I think 1.5" wider overall (.75" per side) but Philo should be able to say for sure. I know it's not much extra width but look at your bedsides if you are doing this.
Also I believe the van ones are 28 spline whereas the bronco ones are 31 spline.
Van rears are 28 spline, and require a bit more work to put in. They also use unequal length axles so that means two spare axles.