In normal weather, Jackson, WY (S. of GTNP) to Boulder is driveable in one day, but it is a long day. With 2 nights in Jackson, you can get a taste of both parks. There are places near Moran, WY (about 30 miles northeast of Jackson) which would be a little more central to both park as well as being on an easy route to Colorado, though still a long day.
From Salt Lake City, accidents and construction aside, I-80 to I-25 should get you to Boulder faster than dropping down and picking up I-70 and going by Grand Junction. Though for speed, I would take either of those routes instead of US 40.