My guess is that the roll pin is broken and the arm is spun a bit on the rockshaft. Since it's a hydraulic lift, it could actually be the one on the hydraulic cylinder side (right side) that's broken and spun, letting the left arm drop lower than the right arm that's spinning. I've seen it happen both ways. There's no real good way to check other than trying to drive out the roll pins and see which one moves. If one moves when trying to drive them out, it's aligned correctly, and you can work on the other side.
First, try to drive out the roll pin from the top, but I'm betting it wont' budge (because they're broken and not aligned). You'll probably have to drill out the ends of the roll pin that are in the lift arm. I'd start with drilling out the the top one, as you may be able to skip drilling the bottom and just drive it out from the top once it's re-aligned. Then, re-align the arm where it's supposed to be on the rockshaft and you should be able to drill straight through to get the middle section out of the rockshaft. Again, if you get it re-aligned, you may be able to drive out the middle and bottom pieces, but I've had to drill them out in the past.
If you get it drilled out and aligned, you can put in a new roll pin and it should fix the lift issue. If you decide to go with a bolt instead of a roll pin, get a Grade 5 or 8 bolt, as standard bolts are too soft and will just do the same thing.
I'll warn you, this isn't an easy job. It will take a lot of patients because those roll pins are hardened or spring steel, and they DON'T drill easy! That plus being a confined work area, make it a real PITA! Get some good drill bits and take your time.