US87 has a broken concurrency in Casper, WY. At Hat Six Road (WYO 253), it should stay concurrent with US20/26 as far as where it breaks off of Yellowstone Highway to join I-25 at exit 186. The exit ramp is kind of strange there. (I actually think it stays concurrent with US20/26 at that point too. I'll make a point of driving through it this afternoon to double check if anyone has any questions.)
It's one of those super concurrent stretches of road that seems to crop up here and there. (Wisconsin, I'm looking at you.)