This will be the biggest pain in doing a V-8 swap into an ST.
Passive Anti-Theft System (SecuriLock):
98~2001 Explorers are type B PATS which uses a standalone PATS Module. All 2001~2003 STs manufactured after 7/24/00 have type E PATS which has the PATS Module functions integrated into the PCM. (If you can find a rare early 2001MY ST manufactured before that date, it should have the same B PATS as the 5.0L Ex.)
For the 2001.5~2003 Sport Tracs that came with type E PATS: if you just swap in the 98+ Explorer 5.0 and its PCM/calibration, PATS will no longer work and the engine will not start. To correct this, you have 2 choices:
Find a way to bypass/disable PATS; or
Install and wire the PATS Module and PATS transceiver from the 5.0 Ex into the ST. You would also need #1 transponder keys to match the type B PATS components.