Sorry to resurrect an old thread, but this is something I know enough about to be dangerous. =)
The airbag system is often coded to the VIN of the vehicle, and the VIN is encoded into the ECU. These are anti-theft and public safety "features", designed to make it more difficult for you to self-service these systems. Putting in a different ECU or a different airbag diagnostic module without a dealer may cause discrepancies and might cause you to fail a DMV, state or dealer inspection.
If you change the airbag system from a donor vehicle to your own, you may or may not run into problems. I live in Florida, a state without any inspections, so I have no qualms about doing my own repairs. I am also aware that some state inspections that will not fail your vehicle for airbag issues either. The vehicle will be noted as having non-functional airbags which prevents you from receiving the insurance discount until a dealer or properly equipped repair shop "fixes" the system. For a dealer inspection for the purposes of a used car sale, it would then be required that you to notify the buyer of your vehicle the airbag is non-functional and this will obviously devalue the vehicle accordingly. Failure to do so could cause huge legal liability if the new owner is killed in an accident where the airbag could have or should have deployed.
Another issue you can run into is that I have also heard that to even register a vehicle, sometimes they will attach a scan tool and refuse to register the vehicle if the ECU VIN does not match the vehicle VIN. In some older cars, if you could dump the raw ECU program to a file and edit it with a hex editor on a computer, you could change the VIN to that of the airbag donor and re-flash. This would cause the SRS to work just fine, but the repair will be detectable.
So much for that clear Carfax! This may come up in an inspection as well. It will look suspicious if the vehicle VIN they are reading is totaled in a junkyard, and you will have explaining to do. Now you are back at square one because once you mention you replaced the airbag systems yourself and hacked the ECU to make it work, you will likely have to make that same trip to a dealer or shop.
I am not sure if all Ford SRS systems do this, but I know that most import systems do. The computer systems they use for reprogramming are actually extremely simple, but they seem to specifically sell them at a price point that would exclude most small-time car thieves and DIYers but not be a burden for repair or body shops.
It is also not difficult to disable the driver side airbag on a Ranger, if I recall it takes 3 small wires, a relay and either a momentary or push-button switch. Press the button and the airbag is immediately turned off for the duration of that drive and the SRS yellow light will be illuminated on the dash. You don't have to worry about the passenger side because Ford already provides a disable switch and associated yellow light already.
My own justification to prefer a push-button rather than a switch would be that I don't want to have to remember to turn airbags back on and want to press the button for temporary disable at specific times. These would include a small fender bender that might become a huge expense if the airbags deploy. In a major accident happening unexpectedly like a car going over the median and hitting you head on you obviously have very little time. You would not have time to even think about reaching to press the button. This gives the choice of no airbags while ensuring you would not be able to disable the airbag in a situation where it would be "needed".
Be aware that adding either a switch or push-button is highly illegal unless you have a exemption from the NHTSA issue and on file. This exemption paperwork must follow the car throughout its whole life, and ugly stickers must be affixed to the dashboard of the vehicle.
You can access form No. 2127-0588, the
NHTSA Request for Air Bag On-Off Switch here. The actual law is over 200 pages it seems, but the government provides a pretty complete
Air Bag Deactivation FAQ.
If you don't go through this process, you may be found guilty of a criminal misdemeanor offense and/or fined a large amount (something like a minimum of $3500-$5000 depending on where you live). Insurance may not cover you at all in an accident if the airbag fails to deploy because of your "unauthorized" modifications to the system. If you are on record as having illegally tampered with a vehicle, this may prevent employment in the automotive industry or cause you to lose your current job, depending on the policies of your employer.
In 2004, West Coast Customers was fined $16,000 for an episode of Pimp-My-Ride where they actually filmed themselves committing the "crimes".
On the other hand, while I would of course
never commit such a
heinous and unspeakable crime of not first receiving government scrutiny, approval, and authorization of, it is very easy to do undetectably.
Welcome Visitor! Please take a few seconds and 
What for? He didn't remove the steering wheel, just the airbag from the steering wheel.
