Just get a longer and use some NON permanent thread lock? Just a little. All my servos (same setup as yours-650 on throttle and steering on revo) ave not had a problem with backing out screws. I've cut the wire in half on one, and that same one wouldn't work, so I let it sit for about 2 months, took it apart and put it back together, no changes and nothing new, and it worked. I like JR!

Oh and if you're not now, get some hex screws for that truck! You'll be able to get them tighter.