All 4.0l Rangers got 8.8" axles, and all 4x4 super cabs did as well but 28-spline axles, 2002 and up FX4 got 31-spline
Explorers got all got 8.8", all 31-spline axles
31-spline is a slightly bigger axle.
I doubt you would blow a 28-spline axle with a V8, but if asking which is heavier duty then Explorer 8.8" would be that.
If you could get an axle from a Ranger FX4 Level 2 it would be a Torsen limited slip with 31-spline, it used gears instead of clutch plates for limited slip.
That would be a step up from Explorer axle, and would of course bolt right in since it is from a Ranger :)
Explorer axle can be bolted in, but you will lower the rear about 4", read here: Explorer 8.8 Rear End Swap – No weld
More popular is welding new perches and shock mount to stay at the same height: The Ford Explorer 8.8-Inch Axle Swap
If shopping around for axles are wrecking yard have a look here, Door sticker can be helpful: http://www.therangerstation.com/tech...le_codes.shtml
Because you have a 4x4 you need to match your gear ratio or you will have to swap front axle gears as well, so look at your door sticker to get that info.