Things You'll Need
Instructions
 
Open the Willowtree editor. 
Click the "Quests" folder to open the available quest saved game data for Borderloands. 
Open the "PT2" folder. 
Delete all the quest logs except for the following: 
Z2_Missions.Missions.M_FindSteele 
Z2_Missions.Missions.M_FindTheVault 
Z0_Missions.Missions.M_AccessStores 
Z0_Missions.Missions.M_FixHealthVending 
Z0_Missions.Missions.M_IntroStateSaver 
Z0_Missions.Missions.M_KillBandits 
Z0_Missions.Missions.M_KillSkags_Zed 
Z0_Missions.Missions.M_MeetAl 
Z0_Missions.Missions.M_RepairClapTrap_02 
dlc3_MainMissions.MainMissions.M_dlc3_GetLoot 
dlc3_SideMissions.SideMissions.M_dlc3_GetLoot02 
Place a check next to the "Reset Playthrough 2" and "Enable Dynamic Scaling" boxes. 
Click "File," then "Save" to complete resetting the second playthrough.