You could pay a doctor to heal you, which fills up all your limb health meters. There may be a way to select the limb in your Pip Boy and apply a stimpack to the selected part, like Fallout 3, but I haven't tried that method yet in this game.
In hardcoe, you can't apply stimpacks to limbs, only Doctor's Bags (from the Stat screen in the PIPboy).
I've found that sleeping in a bed owned by you (rented motel room in Novac, for instance) gives you full healing (including limbs) as well as the well-rested XP bonus. This may be a glitch, as random beds give you NO healing in hardcoe.