Code "XA" was used in all 98-02 and some late 97s and early 03s. "XB" was only used in 2000, as was "XA". Almost every 4T65E in 2000 had two different options codes as there was a minor valve body change in that year and that typically would distinguish the difference. Interchange books often say it must be replaced by the same but that has more than proven to be untrue. 98-99 will have a threaded end style cooler line fitting while the 00+ transmissions all used quick connect style cooler lines. You can use a 00-02 in an early application by using the correct cooler line fittings. Electrically all 97-02 GTP/GS model code- HK (97 only),XA,XB transmissions will interchange just fine into another GTP or GS and wiring/pcm arrangement. You can also use a larger bodied car trans such as one from a supercharged Bonneville. The 99/00+ models had an internal mode switch but it wont get used, you just put the external switch and range lever from your GTP trans on top of the other trans.
