On my 2000 f250 diesel, it seems like the ac blows barely on the max ac setting. I swear its gotten worse over the years. Is it the fan? something else?

 

Probably, the evaporator has gotten clogged with dust or, perhaps, a critter got in there and made a nest.

Pull the blower out and inspect the plenum and evaporator.

 

Though just a "shot in the dark", try this:
https://www.ford-trucks.com/forums/8...ml#post7724237

Pop

 

Is it only in the MAX setting or is it just fan speed in general?

If it's just in max, I'd go with something restricting the cab suction.

If it's the fan in general, it could be the resistor bank (reduction circuit) that controls fan speed or the blower itself is getting old and weak.