1) Using Advance Adapters special extension housing and other kit parts, you can run a stock BW1354 Ranger/Explorer case behind the Mountaineer's 4R70W transmission. This may also allow the use of matching stock four-door Ex driveshafts that are compatible with the 1354. It should be about $600 for everything from Advance not including the transfer case and driveshafts (if necessary).
2) Many full size Ford manual cases will bolt right up to the factory extension housing on the AWD 4R70W (six bolt and 31 spline). In a 5.0 Ranger, the clearance to the LH frame rail is very tight. In a 5.0 Ex/Mountie, the frame rails are spread apart more so there is more room. Different driveshaft lengths will be required.
Depending on the years of the truck and swapped transfer case, there can be complications with getting the speedometer working. This can be true with either of the above options in a 96~97 5.0 Explorer/Mountaineer. 98~2001 4-door Explorer/Mountaineer do not have this problem because the speedometer signal comes from the rear axle ABS sensor by way of the 4WABS Module.
Here is a How-to for installing a full size 4406 in a 5.0L Explorer/Mountaineer:
I have a manual BW4406 from a 2003 F150 behind the 5.0L/4R70W in my Ranger: