I've been plagued by the shoulder screws backing out, the ones at the top of the rear shocks on my 2wd slash (Traxxas 3x12mm Shoulder Screws (Ultra Shocks) ). Tried making them tighter until i stripped it and had to replace the shock tower and still I had the problem.
I just found spare parts in my kit that fixed the problem nicely. Two 3x25mm screws, two 3mm locknuts, and two plastic shoulders from the shock parts tree that came with the slash. Won't back out, and not too much trouble to dismount, but you could also use 3x20mm screws and no locknuts to just get a lot more thread into the shock tower plastic.
I don't know what the plastic shoulder is supposed to used for on the shock, it's not on the exploded diagrams, but it fits perfectly into the shock cap hole. The shoulder is at the upper left and lower left of the parts tree in the photo.