I thought for sure this would be a simple "you soldered this wrong" type query. Does it seem a bit weirder than that to you?
The DC motor control is really separate from the servo control, It would be hard to imagine a soldering problem on one that would affect the other. But motor noise on a servo signal is a very real problem (I have one sitting on my bench right now that drives the servos nuts).
An analog servo doesn't know the difference between a noise spike and a servo control pulse. A bunch of narrow spikes like you get from a brushed DC motor can be be interpreted as "slam the shaft against the low-limit stop as hard as you can". The
I do have a .01 uf (?) capacitor across positive and negative on each of the motors as per the instructions. Is it possible that is not enough?
Sometimes that is enough, but not always. Judging from what has been posted here, the motors on the Make 4WD platform are particularly noisy.