Confirmed upgrades on a domestic route are, indeed, going to be tough to find. I clicked through a few dates coming up, and all I found were two SMF-DEN flights on January 31; the 5:18 AM departure could be confirmed for one passenger, or the 3:42 PM departure could eb confirmed for up to six.

But you should have no problems finding F award (not upgrade) inventory. You may not like the value -- for April 22-27, the current price is 71,800 miles RT for most of the nonstop flights -- but it should be possible to book if you want. (The price could change at any time, BTW; they're not fixed, and UA can and does move them both up and down. The minimum I'd expect to see for a domestic F ticket is 25K one-way / 50K round-trip). In your original message, you seemed to suggest you were having a hard time finding awards also, but that should be straightforward, as UA offers every seat as an award if you're willing to spend enough miles.