Hy jmh, welcome, please remember to tell us what year your truck is when you ask questions. If you were to leave the hydrovac out of your brake system, you would have no decrease in brake capacity, you would have to generate more pressure with your foot however, The hydrovac can be rebuilt, and you can find them rebuilt on "E" bay among others, as for the rear axle, yes you can swap it for one with an electic two speed, but why don't you determine what is wrong with the one you have and repair that?
