Truth be told, during the last game that I'm on I've never went to grind for XP or gold, but for items, and even then I got what I needed after 1 or 2 runs.
What comes to mind is:
Gorx Lair - level 6/8 - I used to like this spot for farming Firesalts, now I think fire cave in Tremadans tower might be better
Merdxan Cave - level 8/10 - Nevermelt ice, Icewolf pelts
Kurgua Cave - level 12/14 - Ivory ring and Mystic bone mace
Mausoleum - level 15/18 - Ruined dragoon boots, Tolassian chain armor, Scrolls of wisdom
Crimson hills - level 17/18 - Corrosive weapons, Ring of the frozen heart, Golden apples, small diamonds
Abbey of Saint Admus - level 18/20 - Cursed fang, Mystic sandals
Sunken citadel - level 20/22 - Ashen leggings/boots/helms, Leggings of Shadows, Blacksilk leggings, Potions of Mana, Elixirs of Galade
Forbidden pit - level 20/23 - Potions of Mana, Emeralds, small rumor boss loot
Ark of Lothasan - level 22/24 - Ring of the star traveler - by the time you get enough of those, you'll find everything else of value

Icemist underlevels - level 22 and up - big rumor boss loot