Marder wasn't back-heavy because its armor was quite thin. Weight of the engine was enough to counter the weight of that "turret" in the back (which isn't a turret anyway, it's a non-rotating fixed superstructure). A typical design for a German lightly armored self-propelled gun (Nashorn, Wespe, Hummel looked like that too).
We've got a nifty shuffle anim on our ewebs which might be of some use, speak to your local garden ornament. Our eweb guy stands while yours is crouched, but the logic should be the same. The animation for a crouched guy would be a bugger though, methinks.