You've heard of the X-treme Off-Roader, but have you heard of the Super-X-treme 6x6 Off-Roader with RWD-steering?
Two 42099 sets and about 1400 pcs used, 5 pounds and 21 inches in length. In total two technics hubs, 3 XL motors, 2 L motors. Everything controlled via BrickController2 and a Nintendo Pro controller, works extremely well. A very good crawler with lots of torque, it can go up and down sofas without little effort. Common 42099 mod performed where output gearing is switched around for more torque. Standard diff between all wheel pairs but the two rear axles purposefully without central diff, two XL motors drive the four rear wheels together for high torque. Very simple building instructions: Build one complete 42099, then build an extra front axle with steering and motor and attach in forward facing direction aft of rear axle. Quite a lot improvisation here but doable with time to play around. Technics hubs moved down and to the sides for better weight distribution, easy to change batteries without removing the hubs. If you want fixed drive between the two rearmost axles the easiest is to flip one differential inside one gearcase so it spins the opposite way, that way you can have a straight axle between two rearmost wheel pairs. With BrickController2 you can fine tune how much the rear axle steers in relation to the front axle, currently it's set to about 50%. Had lots of fun with this one and drives great!