Look at your Chord spelling for the D7, DF#AC, the F# is the major 3rd, so you know by that, you must be in a major scale, so again look at the chord spelling, there are 4 notes and you only need 3 more, think of a scale that contains those 4 notes, times up, GMAJ, GABCDEF#G, play that scale starting on the 5th, DEF#GABCD, I believe thats D Mixolydian, stangely it has a 7th kind of sound, and works pretty good for those country rock type o things, I believe you'll enjoy it's delicate bouquet, and nutty yet playful flavor, I've used up all my limited mental abilities now, so I must recharge, .... . . . . . .