Do you ever consider taking a mono-ski or snowboard, and letting the hounds carry you uphill? I just love that image.
What it sounds like to me is that your feet are getting a little wet. True? Otherwise Im not sure I can account for feet getting all that cold, even on a chilly Montana day, on a descent that probably doesnt take more than an hour.
So, first thing Id do is just make sure your feet are staying dry. Use a good waterproofing treatment on your Sundowners like Nikwax Aqueous Wax ($8.50). Then maybe upgrade your socks. If you arent already wearing wool outer socks like SmartWools Hiking Socks ($18) then get a pair. They have a wider temp comfort range than most other socks.
You can do a few things to make your boots a little warmer. Neoprene socks is one. They do breathe, so your feet wont get all that wet from sweat. Cabela'ss sells a 2mm pair for $13. And add some Insulator Insoles ($9), which provide better insulation across the sole than the standard insole. And lastly, spray your feet with anti-perspirant. Seriously. If your feet are getting wet from sweat, then do something to short-circuit that process.
Your last resort is to find new boots. Vasque Sundowners ($180 in GTX edition) are good boots, but not real heavy. And of course not insulated. Maybe you could try a pair of The North Faces Four Shadows GTX boots ($150), which are a decent hiking boot with Primaloft insulation. Warmer than what you have now!