The BII axle is 3" wider then the Ranger so it will increase your track width in the rear.
Also teh BII uses different shock mount locations, a different sway barm and possibly different spring perch locations.

I cant remember for sure, but you may need to cut off the shock mounts and spring perch's then weld on new spring perch's and shock mounts :)

If you are going to go to all that trouble you mine as well upgrade to a 8.8 rear from a 91+ Explorer or Ranger.....

Oh and MAKE sure you check the gear ratio if it's a 4x4!!!!!!

You can read more about the different axles at:

www.therangerstation.com