I assume that you have verified there is nothing obstructing the airflow?

I have found that mice will even make a nest in the air ducts during the
winter.

FWIW
David

You might have undesirable resistance somewhere in the motor circuit. The
proper test would be to use and amp meter to check for correct current and
perform voltage drop tests on the circuit to find any excessive resistance.
Another possibility is that the duct work is faulty or one of the air damper
doors is binding or leaking air. If the motor has the correct current draw
then a close inspection of the plenum and duct work would be in order (not
an easy task).

Also, you said you have a new A/C system. Does that include a new
evaporator? If not, the old one might be clogged with debris and blocking
air flow, or if a new one was installed, they may not have sealed the case
properly.

just a thought, a damper door is malfunctioning, the *wrong* fan motor,
or, and this can happen,

a leaking evaporator coil that has collected dirt overtime, leaves,
debris obstructing air flow across the evaporator coil.

if the ac is working at optimum, and the exiting air flow is not up to
standard, look for a lack of air flow across the evaporator coil.

does the suction line sweat good next to the compressor when the unit
has run for a while?

what is the temperature coming out of the register, is it 40°?

I have the same exact year and model of that van.
The fan motor (OEM) is normally a lot more powerful than what you describe.
Nontheless, the heat load on that van is large, and it can be difficult to
cool
in hot climates.

On my van, I have burned out that crappola resistor once.  I found that it
doesn't
burn out on its own...the fan is drawing too much current, possibly
intermittently.
But you say you have replaced both the resistor and the fan motor.  Did you
use
an OEM motor or a local parts house one?  Believe it or not, Dodge quoted me
a
cheaper price for an OEM than the local parts houses.

Next, the switch on the dash of my van is touchy.  I haven't replaced it
yet, but
need to do that.

Others have posted about the other typical causes of low fan output, so I
wont
go there.

I found my van was hard to keep cool, and I kept running it in Max mode.  As
you
can imagine, it would freeze up pretty quickly under those circumstances.  I
applied
window film to lower the heat loading, and it has worked very well. I keep
cool in
all but the very freaking worst of weather now.

I used the 3M film available at places like Walmart, but was VERY careful in
installing it.  I have had it on the van for about three years now, and you
can't tell it isn't a factory job...no bubbles, no disbonding, nothing.