EDIT: Remaking the first post with new locations and information, would love to make the thread renamed to something related to instance solo leveling.
So being two levels higher than a dungeon lets you have the legacy buff which allows you to run it fairly fast and kill pretty easily, the idea is to do dungeons which still reward a large completion bonus, are fast to clear and to exit. These dungeons are done on normal mode to allow you to reset them at the end.
The general idea is to enter dungeon, clear the main objective, leave, reset dungeons and repeat.
Level 81/82-86 - Forge of Souls, easy to clear at around 3-4minutes per run, after killing the last boss enter the Pit of Saron instance portal, leave reset and repeat (remember to do the quest to access the Pit of Saron portal at the end!)
Level 86-88 - Stonecore, easier than Tol'vir for the level range and still gives competitive exp on completion, so same idea, run in, kill boss, leave and repeat, can make a small fortune in gold for the runs to!
Level 87/88-90: - Lost City of Tol'Vir, extremely fast to clear and to leave, at around anywhere between 4-5mins per run, 36g for completion and a estimate of 40-50g in blues/greens per run, easy 2k-3.5k gold after you reach 90. Big thanks to olgafardin for finding this one!
https://i.imgur.com/mlHT8tm.jpg - FoS
https://i.imgur.com/25PyWni.jpg - Tol'vir