Best money spent would be to put the truck back to it's intended configuration.
The cost of OEM replacement gear has come way down in the past couple of years.
I chose to go even less expensive and used the replacement shocks and air compressor from
Arrnott Industries.
With 30,000 miles on my Arnott gear, I have absolutely no complaints.
Your mechanic will continue to chase his tail until he understands how the system works.
First mistake someone made was to replace the original shocks with any shock that does not plug into the truck's computer system.
If non-OEM shocks are installed (like the Arnott shocks which come with this bypass device), then some sort of bypass device must be included that are designed to simulate the presence of a shock to the computer system.
Otherwise, you will forever get the error message in the DIC.
Next, there are sensor devices mounted between the truck's frame and suspension that report to the computer the current height of the truck.
OEM air shocks will RAISE the rear end of the truck a certain amount over the height you will get with either a blown out air shock OR and a replacement non-air shock.
My truck's rear end sits a full inch higher when in it's normal height as opposed to an inch lower when I had the blown out air shocks still installed.
If your mechanic can not find a way to adjust those height sensors to find the "sweet spot" that the computer is looking for, you will always get that error message in the DIC.
Your 3/4 ton or whatever shocks you have installed are not going to "lift" the rear of the truck to satisfy those sensors.
They are not designed to do that.
Pulling fuses and cutting wires isn't going to get you to where you want to be.
My advice would be to put the truck back to the way it was designed.
If money is your determining factor, then do it one component at a time.
I would say start with the air compressor so that you are not going to be driving around with new air shocks for a while without them having any air in the bladders.
You could just leave the new compressor sitting in it's box until you have a chance to gather up the new air shocks.
Once you have all of your new parts bought and paid for and sitting there, spend a day putting your parts on and say goodbye to the error message in the DIC.
If that is not the direction you want to go, that's cool too.
You or your guy will have to determine how to satisfy the computer in regard to the sensors located at each shock (hint: there are guys on other forums that have determined what resistor is required. Think Cadillac and GMC Denali.)
Then you guys will have to locate and rig each of the ride height sensors on the frame to fool the computer into thinking the ride height is correct.
One thing I am not clear on is if the air compressor is monitored and has to be plugged into the system in order to prevent an error message.
If the compressor is not monitored and the computer is kicking error messages only because it does not see the expected shocks and it can not achieve and maintain it's intended ride height, then the hacks I mentioned above should get rid of the messages for you.
Me, I'd fix the truck properly (I did) and be done with it.