What temperature does it drop to along the Karakoram Highway?

October is a great month to be in NW Pak, the colours are amazing, however it is Autumn and temps will be starting to drop a bit, especially at night, when you will definately want something warm to wear. Depends a lot on where your going and how your travelling, I was in Baltistan in Oct and the days were sunny and warm, but the nights were cold,up at the Khunjarab it will be considerably cooler. If you don't have enough warm cloths, you can always pick up a local woollen blanket 600-1000 rps, just the job !
UB.

During the day I'd be fine in a sweater and jeans?

Jeans & sweater is enough in the day time but you need something warmer in Karimabad & in upper Hunza like Gulmit & Passu at night.

Its warn at daytime but pretty cool during night, average a comfortable, but as i said some places will be very warm due to barren rocky mountains...