I took the AP US History exam junior year after taking a two year course. I was lucky because having US History split into two years allows for a lot more depth as well as a whole month(!) of review for the exam.
Last year, the DBQ was about Nixon, so don't automatically assume that you don't need to learn modern era stuff. Still, don't worry about it. If you have a general grasp of the events (Vietnam, the oil crisis, etc.) you'll be fine.
My advice to you would be to practice. Take as many practice tests as you can get your hands on. That way, you'll get used to the multiple choice questions, as well as discover what you don't
know. After checking your answers, look over every question you weren't sure about. Write out the event/person/election/concept, look it up, and take a few notes about it.
Now, the most important thing you need to get a 5 on APUSH....
This book will save your life.
Read through it and highlight to your heart's content. It explains everything
you need to know about US History, and it's easy to read. I owe AMSCO for my acing every APUSH midyear, final, and the AP test. Reading through the book really helped me review the material and connect the events better than just looking over notes or vocab. Tell your friends, share the wonder that is AMSCO!
Good luck on the exam! An important thing to keep in mind is that you don't need to get everything right. I don't know the exact percentages, but you probably only need to get 60% of the multiple choice right and write fairly decent essays. Don't stress!