Hello!
Folks mostly glue everything in place. To make sure everything stays put it's not uncommon, especially in case of figures, to drill a hole poth in "the ground" and the leg of the fig (or a wheel of the vehicle) and use a short length of wire in both holes to glue them together.
In your case you could make the ground a little uneven and show off how the suspension articulates. Also, please don't forget to show the vehicle tracks in the ground, so that it's clear it didn't teleport to the spot it's now in.
And - you didn't ask how to make sand in scale - do you have that worked out? If you just glue regular sand to the surface it will look to coarse in scale... Exactly, scale - what do you have in mind - 1:72, 1:35 or something else?
Good luck with your diorama and have a nice day!
Paweł