Hey Dustypumkin,
Like you, I was worried about my pumpkin plants when I saw that the stems were all cracked and dry and generally unhealthy looking. However, there is absolutely no need to worry. As long as your vines are vigorous, obviously water and nutrients are moving through that crusty stem. As plant cells age, more cellulose and even a little bit of lignin gets deposited in their cell walls. This gives them a hardened, woody-like texture as well as a lot of strength. That is what is occurring in the stems. They are less pliant because their cellular composition is becoming more akin to that of wood and less like flexible, green, wet pumpkin vine and leaf tissue that we're used to. Your plants will be fine!

Good luck,
Joe