Taking a road trip including a drive from north to south on the million dollar highway in mid September. Any good national forest spots to spend the night? Ideally somewhere between Ouray and Silverton. Have a 4x4 and used to public land camping. Thanks for any ideas!

Lots of spots.  If I am down there later this month, I'll try to get you some gps points.  It's very remote, so just about any forest service road will have spots.  Might want to check freecampsites.net

Between Ouray and Silverton if you want to be close to the highway I'd recommend Ironton for dispersed camping.  If you don't mind getting off the highway and getting up a steep switchbacking high clearance road (I did it in a Grand Cherokee) and camping in a stunning alpine setting by s lake I'd go up to Clear Lake just northwest of Silverton.  An easier to access spot with limited spots is Little Molas Lake just south of Silverton. There's plenty of places to dispersed camp down there. Enjoy! That area is spectacular.

Great, thank you both for the suggestions. I have a 4Runner so hearing you did it in a grand Cherokee is helpful!

Ridgway State Park is nice, might be too developed for your tastes, though.  Camping up on the hill is nice, great views.