Are you asking if you can swap a 4x4 trans into a 4x2 truck or convert the 4x2 trans to a 4x4 trans?
The output shaft is different between the 4x2, and the 4x4 trans, also the tail-housing, and transmission pan as well. You would have to tear down the 4x2 trans to install the 4x4 output shaft. Probably be cheaper to find a good use 4x4 trans.
Here is a link that may help.
http://www.rangerpowersports.com/forum/showthread.php/209598-how-to-convert-your-5R55E-4x2-to-4x4