I know I don't need one at all.... but I have the oportunity to get one for about $60, so why not? 
My problem is, they're listed(yes, 2 of them) as early pick-up 31 spline, and the other is 29 spline. Mike's write-up says his choice was from an early Bronco, and I'm not sure if they're the same. I asked the seller about r&p and he thinks they're 3.xx. They're a couple hours away, and I was hoping to find if it will work before I wasted a trip.

from 4 Wheel Off Road magazine, December 2006, page 124:

Ford 9" Rear
Round, with front dropout 3rd member and no back cover
No C-clip, 3 pinion bearings (2 is typical)
Came in fullsize Fords
Good for: High performance and off road vehicles. Consider it for 1/2 ton trucks that will have up to 35" tires, or skinny 37's.

so, a fullsize bronco could definitely have one. im not a ford guy, but i think they should all be the same except for the number of pinion bearings and splines, but dont quote me on that because im not sure. also make sure it is a 9", not a 8.8", which was available in explorers, f150's, ranger's, aerostar's, and broncos, and has 31 spline axleshafts and disk brake option, but isnt near as strong.

The reason for using an early Bronco was for the width of the axil.  The Early Bronco is only a little wider.  Measure your backing plate from side to side and then call you guy up and have then do the same.

If there with in a couple inches, go for it.  Other wise your wheel base will be too wide.

Eric

$ 60 for a ford nine is great if it fits.

Backing plate to backing plate is easiest to measure with the wheels on  but WMS ( wheel mounting surface to wheel mounting surface / studs to studs ) is the best measurement to check for compatability. Its a minor point but just my 2 cents...Gary

if there early pick-ups there not the same. they should be 28 or 31 spline.  stan