It's really unfortunate, but only 2 random quests can boost your legal reputation, and both are merchant quests. One is where you are accused of stealing a gem, and if you complete it successfully you get +10 rep to that region. The other is an escort quest, where you have a 1:4 chance of getting the opportunity to hand the person over to the city guards. Really, more quests should affect your legal rep; shame they don't.
The irony is that I just spent literally hours today doing a merchant quest in a dungeon hoping it would help. I couldn't find the darn harpie feather and left the dungeon figuring at least I would have a good bit of gold. I got a "note" from a courier just outside the dungeon that said while I was in the dungeon, the fighters guild asked me to find the first guy they sent in there. So I went back in. Pretty quickly, I found the harpy feather. But I could not for the life of me find the lost fighter's guild guy. I finally gave up on THAT figuring at least I finished the merchant quest. I had so much gold on my person that I couldn't pick up another coin. Well, all of this took too long and I failed BOTH quests. I netted 45k gold so it was worth it but still...