How many hours on it?
As you may know all 3 of these circuits use the same pump and the same relief valve, the only way the pressure to one of the 3 circuits would be low is if the foot pedal is not fully stroking the spool on the valve. You can pull the cover between your feet and verify the spool movement, it should be equal to the spoold for the tilt and Aux.
Now you say the pump is weak. Was it flow tested under pressure at a shop to determine this? If it is weak you should notice its performance drop as the hydraulic oil temp come up to normal (130- 140f in my Canadian climate).
Assuming the spools are fully stroking, my guess is it takes less pressure to move the bucket dump cylinders and the grapple then it does to lift the boom, and thats is why you see the limitations of the pump with the boom function.
You can hook a guage in your aux and the relief pressure should make it to 2500-2600 psi.
If it makes 2500 cold but not hot then your pump is weak. If it won't make 2500 cold, then the pump is shot.
Of course the proper way to verify this is to conect your aux hydraulics to a flow tester ad see what volume of oil the pump puts out with 1000 psi load on the circuit. it should be 15.5 gpm with the engine at 2900 rpm.
If the temp does not have any bearing on the pressure, then it could be you have a weak relief valve, these are not adjustable and need to be replaced.
More info is here.
http://www.mediafire.com/?ov9zqt7xwtl
Ken