The slider lock is there to lock the doors in a slightly open position for storage, not for locking them completely closed. The door latch should hold them closed on bumpy roads, at least they never failed on me, and I've been on many rough roads with my TT.
If the lock doesn't engage, try pulling upward on the bottom of the door a little so the latch can slide into the hole in the frame. But still, it's designed to hold the door open a little when locked with the slider.
Maybe the door latch isn't engaging completely, and that's why it opens on the road. Again, maybe slightly lifting the door will allow the latch to fully engage.
And did you look to see of your hinges are starting to crack?
2011 Dodge Ram 1500 HEMI Quad Cab
2011 Jay Flight 26BH