Your issue maybe you are not leeching enough, if you do have decent leech, then your phys dmg maybe too low. This is if you carry on with Frenzy barb.
What is your gear, what act / difficulty, what merc, what skill placement and where are you putting your stats?
Have you tried just running act 1 bosses (norm) for low set drops like hsarus (belt / boots) or pieces of angelics (pref 2 rings, ammy and armor), any runes, or runewords made?
Also, remember what monster types appear more often in areas, like Crypt / Mausoleum have a lot of undead skellies, so leech doesn't work.
Work around areas to your barbs strength with what you have... I don't play D2:R, however in LoD I preferred the Rogue merc to Nightmare Act 3 before switching to Act2 merc, also you have more variety in Merc abilities I believe, so use what you think works for you.
I know this did not help too much, but hopefully something for you to think on.
This was very useful, thank you for taking the time to help!