So, the spec sheets I have seen hav HAEMP hardened listed on them.
What does that actually mean?
It means that the truck is designed to withstand a High Altitude ElectroMagnetic Pulse (HAEMP). One of the secondary methods of nuclear attack is to detonate the missile high in the atmosphere, not on the ground like in an attack designed for physical destruction. The resulting EMP is then able to reach a significantly large geographical area (e.g. most of the USA in one shot).
The EMP generates a rapidly changing magnetic field, which in turn generates a massive elecrical current anywhere that it passes through a "coil of wire" (or other analogous things like circuits with traces in a loop, power lines running in a loop around your city, solenoid coils, transformer coils, etc.). The specific wavelength of EMP expected to result from such an HAEMP attack is especially devastating to smaller-sized electronic devices (such as computers, gasoline engine ignition coils, small wall-plug transformers, etc.). It's anticipated that nuclear attacks would likely start with, or at least include, such attacks since they successfully impact an adversary massively, without many of the negative physical effects. There are currently theoretical new generations of nuclear weapons being studied whose only major effect is the production of an EMP.
For the trivia-minded people out there, this is different than the EMP wavelength created by coronal mass ejections / solar flares, which is much longer, and instead generates huge over-current in the power lines / grid itself. That in turn destroys the large oil-filled transformers used at substations and on power poles, for which there is little manufacuring capacity (currently) to replace them (compounded by the fact that large geographical areas would be without reliable grid power, some estimates are as long as several years to rebuild all the transformers that would be destroyed by a coronal mass ejection the size of the the Carrington Event of 1859, which destroyed a lot of telegraph systems but there weren't yet common residential power systems yet to affect
https://en.wikipedia.org/wiki/Solar_storm_of_1859 ). However, generating huge over-currents in the power grid also destroys many smaller devices that would be plugged in at the time it occurred (e.g. computers, but not vehicles).
Military vehicles are hardened against HAEMP by shielding sensitive or critical electrical components. Like a faraday cage, the currents the EMP generates then flow over the surface/mesh of the enclosure/shielding instead of through the wiring of the component itself. Also, some interconnected devices have fuses, circuit breakers, or other current-limiting devices between them so that generating over-current in one device does not propagate to another. The antenna bases have some of that capability, for example.
