As far as I know, only a Bronco II axle will bolt in. As you found out the spring perch location is different, and the angle of the pumpkin is different also in a Bronco because of the short wheelbase.
I put a ranger axle in my Bronco and found out all this the hard way like you did. What I did was go to Tractor Supply and bought some trailer axle spring perches. They had some that fit perfectly against the ranger axle. I then took a torch and cut the old ranger perches off, ground all that down, and then sat the axle in place with the new perches not welded. Got everything lined up and tightened the u-bolts down fairly snug. I then sat the weight of the bronco down on everything, and then took a jack and jacked up the nose of the rearend till it matched the old Bronco rear. The manual says the Bronco rear is nose up about 9 degrees. The Ranger axle was only like 3 degrees. Once I had that set, I tacked the perches with a welder, and then took everything apart and welded them in place. Then put it all back together. I run this setup daily to work, no problems so far.