i must say, broncos are sweet...when i was growing up my dad had a big ole mother, and we used to take it out and beat it in the farm field...it only started on fire once. It took alickin and kept on tickin thats for sure (it ran over lots of things)
If you lok at a dif center section, you will see that the pinion shaft[the part the drive shaft hooks to] is mounted off center. Most difs have in below the centerline of the ring gear. Now a high pinion axle, has the pinion shaft above the centerline. Not many rear axles are setup this way, but a lot of fronts are. The front axle of the little Jeep cherokees for one. What this does, is give you better drive shaft angles. With a short wheel bace, if you put in to much lift, the angle of the drive shaft will be a lot. This will cause the Ujoints to wear faster. With the High pinion dif, the drive shaft will hook up higher, so give you less angle. Not realy needed in the rear of a Bronco, unless you are going HUGE lift. But handy in a small 4x4 like an old CJ.
BTW that 81 should have one of the BEST axles in the world under it. The 9". ALL kinds of parts for it tohandle almost anything you put to it.
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