The chassis is welded steel channel, along with a ten pound steel block on the back as counterweight. (Did I mention a bit of overengineering?)
There's no suspension, so the axles are just mounted on brackets directly to the frame. Initially I'd used some Chinesium portal axles for an Axial Wraith or something, but later discovered that neither of them were equipped with differentials. That combined with the crazy wide tires made turning unmanageable, so eventually I switched to a different set of axles which could be set up with diffs. Seems like diffs are most common in the steering axle and less so in the straight, but for this setup remember it's the rear axle that steers, and since the front has the dual tires, any improvement in turn is good!
Both axles are driven, shafts are solid brass rod with flats ground on the ends for the U-joints, to a dual-output gear motor. The motor was centered on the first set of axles so I was just using couplers and didn't need U-joints, all lined up nicely. When I switched axles I didn't want to re-engineer the whole thing, so just changed out to the U-joints. At least since there's no suspension the shafts don't need to slip.
Steering is via hopefully-sturdy metal gear servo with aluminum arm.
Left side is rear, you can see the steel block counterweight, and the silver square sticking down in the middle is the bottom of the gear motor. The top protrudes up into the cab.
There's a regular ESC to drive the motor and provide BEC power, and then the electronics go a little crazy. There's a PDB by the battery, because the main lift actuators need their own motor ESC (just behind the cab, yellow and blue wires going in.)
Left side with all the orange and white wires is the lighting Arduino. I have a thing for LED's and blinking and get too clever for my own good, so I'll often do a separate 'dweeno for this. As an example
https://youtu.be/w3ayw7PV4zY
When backing up, the white lights strobe blink-blink-off-off while the orange ones just wig-wag. When moving, the orange ones blink, and if the arm is moving or whatever, the front ones come one as appropriate to warn of possible impact. There's also a switch on the TX to pop on all the lights should you wish to induce blindness or seizures.
Second Arduino controls the two actuators and reads their position feedback, so that they stay aligned. Wouldn't want one pushing harder than the other.