Ramparts and Blood Furnace are both staples for XP. Blood Furnace is arguably better XP/hour than questing once you are L65 or so. I grind BF until L66 generally, with quest breaks for sanity, gear, or a diversion.

If you started at L58, you have some work to do in instances, and I recommend doing BF and Slave Pens until you nab honored with those two factions. If you can't quite do BF yet, you can easily do Ramparts at L63 with a team of 5. You don't have to kill the last boss if he's proving too challenging, just skip him. The second guy is cake with grounding totems-- spread your team out in a circle around him, spam groundings when available, and just nuke him down.

If you RAF'd and went to outlands at 58... well, not a good call. Plaguelands quests are sooooo groovy with RAF bonus XP, there's no point to go to outlands until 60 under RAF bonus. (Although I have done Hellfire at 58 plenty of times on other teams/characters).