Does the loader just drop when you push the lever forward?When you push your FEL lever forward, try it pretty slow and not all the way forward (feather it). If the loader has any down pressure at all (before it goes into float), you should be able to lift the tractor. Try it at lower RPM's and, if you feel the loader pushing down, increase the RPM's with the foot throttle slightly. It's important that you do this experiment slowly. If the loader is lifting the front wheels, try increasing RPM and slightly more pressure on the lever.
I'm wondering if you're going right into float. On my machine, there is a distinct feel when the valve goes into float. I'm wondering if the float detent might be set too low and you're going directly into float. Another possibiity is a restriction in the "down" line of the loader.
How many hours have you put on the machine?
Since you use the tractor mostly for brush cutting, there's a remote possibility some debris may have jammed into the QD's and damaged one of them. I would disconnect all the QD's and reconnect them...just to ensure they're fully connected. I really don't think they're connected in the wrong position as that would show up when you're moving the lever.