Okay here's the deal...I too spent FOREVER in Toxx Forest and Steam Mtns....DON'T BOTHER...Head for Paineel. I don't mean the City of Paineel but the large green newbie area and here's the strat:
==> a Pack Rat spawn's in group of 3-5 usually from the SW corner near where u came into the green area from. All I did was continuously kill the waves of them until a wave of undead rats spawned. Now here's the trick...the undeads don't always spawn where the rest of the pack rats spawn...so you have to keep checking the hillside behind you. Trust me...Kill everything in the zone and keep the "a Pack Rat" mobs dead...and you'll be rolling in the livers mmmmhmmmmm. I killed around 10 or so and got 9 livers!!!!! Great spawn rate and easy mobs.
Good Luck All!!