We do not offer conversion kit to convert XB4 4WD into 2WD. However, if you have XB4'14 you can convert it to 2WD.
Please, check this link
http://www.teamxray.com/xb4-2wd/download/ where you can download the instruction manual for XB4 2WD and here is the list of parts you would need to convert XB4'14 to XB4 2WD:
#362211 + #362221 (you can also use those included in the kit you already have)
#365242 + #964054 - page 19
#361101 + #361261 - page 21
#362572 + #303127-K + #361390 + #303120 - page 23
#364181 + #361162 + #302291 + #302296 - page 25