Are you checking bags or carrying on? Traffic in the DC area is bad, certainly, but it probably won't take more than an hour (and could very well take more like 45 minutes) to drive from DCA to IAD. Uber/Lyft will be $40-60, depending on surge, and you'll want to make sure they have an EZ-Pass Flex so you can take I-66 west in the afternoon (66 is now tolled with EZ-Pass only eastbound in the morning and westbound in the afternoon--it's free if you have two people in the car, which an Uber/Lyft obviously will, but the need the EZ-Pass Flex set to carpool mode). If you don't have to wait for your bag at DCA (and even if you do), this should be pretty doable. There is always the possibility of a traffic disaster, but an hour is enough on a normal day.

If you take Metro, it's about 50 minutes on the trains (if all goes right), plus another 15 minutes on the bus from Wiehle Avenue Station to IAD.