I would wait until late May or June. Later is even better. I was in Yellowstone late last May and ran into a snow storm to where you could not see the mountain tops. Glacier NP in Montanna does not open going to the sun road until mid June. Last year it was nearly July 10th. July or August is better for Glacier NP. There are many beautiful sites to see in the area you are talking about. ND has Roosevelt NP. SD has the badlands, Custer State Park and black hills, Wy has Glacier and many other state parks, UT has Canyonland and Arches near Moab, WY has Tetons and Yellowstone, CO has Rocky Mt. NP, Mesa Verde and Gunnieson, NM has has cliff dwellers of Bandelier, Pecos, and Sante Fe Loretto Chapel. We have visited all these site and they are breath taking.