Welcome to TRS
Fast blinking Theft light means PATS(passive anti-theft system) has turned off fuel injectors and fuel pump, so no start.
It will stop flashing rapidly after a few attempts to start it and you have to wait 20 minutes or so before you can try it again, it is in protection mode since it thinks someone is attempting to steal it.
You can also disconnect both battery cables and touch the cables together for a few second and then reinstall cables, that usually clears theft reset.
'01 Explorer had Type B PATS so will have a separate PATS Module above glove box that needs to be installed in the Ranger
'01 Ranger had Type E PATS, so no PATS module, PATS is in the computer.
PATS module has the unique ignition Key numbers that came with the Explorer when new, so only those keys can be used to pass the PATS test and allow fuel system to start.
Around the ignition key slot is a ring, a wand, it reads the unique number of the PATS key that is being used, it uses a radio signal.
If you have the PATS module installed then try this, I assume it is installed or you wouldn't get flashing light just no start.
Take Ranger key and cover the handle(black plastic) with tin foil, to block its number from the wand, getting 2 numbers would cause a fail.
Place Explorer PATS key next to the ignition key slot, as close as possible, and then use Ranger key to turn it on, you don't have to try and start it, just turn key to RUN, if light doesn't flash rapidly then you should be able to start engine.
PATS test is only when key is turned on, correct PATS key is not needed when driving, only after shutting key off and then back on will it need to pass the PATS test again.
If this works then you can see if you can swap ignition key cylinders to use explorer key in the Ranger, or if you can get it started, then drive it to a Locksmith that has Ford PATS software on a laptop, call ahead and ask.
With that software he can plug into the OBD2 port and add your Ranger PATS keys to the Explorer PATS module's memory.
So doors and ignition match.
Always have 3 working PATS keys available, because with 2 working keys you can add a 3rd PATS key yourself, if you have 3 and lose one then you can add another at your leisure