Ok, so here is the thick and the skinny of the whole thing.
It's not illegal to remove your doors, per se, but if your vehicle is not designed to have removable doors from the manufacturer, and you take them off, then it should not pass the safety inspection. However, some safety technicians might simply overlook that, and pass it, another might say hell no and make you put the doors back on. Which leads into another point I'll make here in a second. So, a jeep with the removable doors would pass the safety inspection if you took them off, but a Tacoma without the doors would not.
Now, as for driving around without doors. There is an code under Utah 41-6a-1601, that is kind of a cover all. It's addresses "vehicles unsafe to operate", which leaves the discretion of unsafe up to each individual officer. So, Officer Johnny Law might see you without doors and say "that Tacoma is unsafe without doors because something could fall out, or a passenger could slip out rounding a corner, or it may cause a distraction to other drivers", or whatever he wants really.. and he could cite you for it as well, then you could go before the judge and plea your case.. hopefully he is a redneck at heart... On the other hand, Officer Lucy Lawless could see the same Tacoma without doors and say "Damm that's HAWT! I want to go wheeling with that rig!" and just wave as you go on by.
One point to remember though, if you remove the doors, you still have to have the mirrors, and if you don't, it will definitely get you a ticket.
Most of the officers out there probably wouldn't even pull you over for it, and of the ones that did, most of them would probably just let you go as long as you were not an ass, a drunk ass, or a stoopid ass with guns and/or drugs and/or warrants and/or a body in the trunk. But then again, there are some guys out there that will just simply pull you over for it and give you a ticket.
Hopefully this clears some things up.