P0128 code

P0128 code just popped up this week on a 2016 JK with 86k miles. A local shop charges $225 to replace the thermostat, but really wants to run a full Dx before repairing, at an additional $125.

What do people think? Should I get the full Dx? Or is it 99% needs a new thermostat? Should I do the work myself? Or better left to the shop?

I am a little nervous that they will charge me extra once they see skid plates and an aftermarket radiator.