a 3.0 Ranger will have a 7.5" rear axle with 3.45, 3.73 or possibly (remote) 4.10's
a 4.0 Ranger can have anything from 3.27's to 3.55's, 3.73's or again, 4.10's (rare) but WILL have an 8.8" axle.
You really wouldn't want to ut a 7.5" with 4.10's behind a 4.0.
AD