You can fit a 7.5" 2wd diff by taking 1/2" off the diff casing upper mount, to fit into the 4x4 beam. The smaller diff won't live long with the power you are putting through it IMHO, you might be better off replacing it before it breaks, so you should be able to sell it for pretty much what a 2wd diff costs. You will also need the 2wd 108mm shafts as opposed to the 4x4 100mm items. It all drops straight on once you have removed the 1/2" slice off the upper mount .

2wd diff si 3.64:1, 4x4 is 3.62:1, so a very small difference, make virtually no difference.