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What year bronco did you get the axle from? if its 1978 or later (fullsize) the axle is going to be about 6 inches too wide to fit properly under your truck. The only way to make the axle you have work would be to have it narrowed ($$$) or to find a 9 inch out of a 1972 or earlier 1/2 ton (f-100) which will be a virtual bolt in and the proper width.
The passenger car version of 9 inch needed the driveshaft to run down the tunnel in the center of the car. To do that, the axles were different lengths, so that the u-joint was centered. Truck rears are equal length axles, which offsets the pumpkin.
The stock 55 rear axle 61.25" hub to hub and the the Bronco rear axle is 65" hub to hub. You could make it fit but you would have to run different wheels front/back.