I am having the exact same problem. I bought dr2 and the s1-4 pass on microsoft store and can access the content just fine in free play, but not in career, and I'm stuck with the original tracks and cars. This also locks me out of any timed challenge that requires an f2 kit car or 4wd up to 2000cccar since they dont show up for purchase in the garage store.
I've tried uninstalling and reinstalling everything from scratch (100gb takes a long time), combing forums, etc. and nothing has helped.
I REALLY hope someone can help with this or that the flat out pack magically fixes things. Help!