It seems to be fixed now.
I had to load up a 64bit livecd and chroot in using:
From there, I changed my /etc/defaults/grub. I had originally been using a multitude of kernel options including acpi=noirq. These settings had worked for well over a year, but somehow broke after the update, I guess. I replaced the acpi=noirq with irqpoll and ran update-grub.
mount /dev/sdb1 /mnt
mount -o bind /dev /mnt/dev
mount -o bind /proc /mnt/proc
sudo chroot /mnt
Upon reboot, the latest kernel loaded without problem. If anything changes though, I'll report back.