Personally I would consider this far to risky.
According to Google Maps, Boulder to Denver Airport at that time of day will "typically" take 40-70 minutes if you take E-470 (which is a toll road). Even if your Uber is pulling away at 5pm, that means you'll be arriving at DIA at 6pm.
Your flight will board at 6:05pm, so literally as you are walking into the door of the airport, your flight will start boarding. Boarding ends at 6:40pm, so you have a little over 30 minutes to find security (not as easy as it might seem!), make it through security (which at Denver can be slow - especially if you don't have PreCheck/Clear/Priority), get to the train, catch the train to (probably) the B gates, and then walk to your gate. Given you have Star Alliance Gold status you will be able to use the priority lane at security which will help a little, but even this can be variable.
Technically that IS possible, but you're going to need everything working in your favor to make it work - and especially given how long the security lines can be at Denver the odds of that happening are low.
United does have a "flat tire" rule which basically means that if you miss your flight they will try and put you on the next one at no charge as long as you arrive at the airport with 2 hours of the departure time of your booked flight - but that presumes there are seats available on the later flight which it's certainly possible there will not be so it's still a risk.
Personally I would not risk this - especially when connecting to an international flight. The Denver/Boulder area is beautiful (I was there yesterday!) and a great place to spend an extra day!