I think that what you are talking about is the phenomenon where, late in a marathon, your leg muscles are fatigued and lose some of their ability to absorb the impact of your footstrike. There is exponentially more force the faster you are running, so it makes sense that you feel it more on the downhills. Theoretically, increasing the number of muscle fibers could help you delay when this happens.
If you are going to incorporate lifting into your training plan, I recommend doing it before you run. This will "pre-fatigue" your muscles, to simulate running on tired legs. For legs especially, emphasize lifts that use eccentric contractions.
Definitely include squats. Anything else that works your quads is good, as they tend to be used as the "braking" muscle when you are running downhill.
You should also include pull-ups. If you are near your proper marathon race weight, you should be able to do a set of 7-10.
Something else you could try other than lifting is just to hammer a long downhill while you are running. This will train your legs to better absorb shock.