Cabin air does not flow well.

The fan works fine and you can audibly hear it change speeds. As the fan speed increases, air flow stays the same. Both cool and hot work. And the filter has been replaced recently.
Is there a possible blockage in the ducts?

 

Possibly if you have had rodents in your truck. The other possibility is your air intake for the cabin is clogged ,it's under your windshield wipers.

 

There isn't anything covering the vents under the wind shield wipers. Besides taking the air filter out from the cabin, is there another way to see into the ducts?

 

Did it reduce flow after the filter change?

I've noticed that some of the "nicer" filters restrict flow so much they're useless. I use 10 dollar cheap ass filters because they're the only ones that the blower can handle. Try buying one of those, the very bottom bargain bin filter.

 

Blower motor might be on its way out.

 

Pull the filter and see if it improves.

 

Our A/C isn’t great; there are several other threads about our blower being weak even on high.

 

Ok, thanks for the feedback

 

best bet is to pull the filter. ORRRRR... the cover wasnt put on securely when they replaced it so you still not getting the full flow.

 

Cabin airflow on the 2nd gens sucks ass compared to many vehicles... even on high it is pretty weak. Nothing much you can do about it assuming your cabin filter is in good shape and the rest of your a/c system is in good order. If you live in a hot region this shortcoming is much more noticeable. The best thing you can do if you've covered all of the above is to get a good quality tint done to your truck.

After reading your post again, despite its flaws you should still be able to tell a difference between all fan speeds, definitely sounds like something is wrong with your setup.

 

Yeah, pull the filter out & see if it improves. Mine works fine on all speeds.
Try to keep it on recirculation, even when off. To keep the rodents out

 

The cover is above the blower
If anything, leaving the cover open should increase flow to the blower.
I’m not promoting this, I am just stating that should not cause the problem.

I have found, that when it is set to Bidirectional (Floor / Dash) the speeds are harder to differentiate.
Try setting it to dash only and see if you can tell a difference.

If that doesn’t help, make sure it’s only coming from the vents selected.
A very small amount or air maybe be felt from the others, but not a lot.
You’ll know if they are working correctly once you feel.

If all that doesn’t work, you might have a leak in the duct work.
Not too long ago, mine spit out some of the foam that goes between the joint fittings.
I never looked into it, mostly because I don’t want to crawl around on the floorboards or pull the dash.

 

Is this a recent change, and if so roughly when?

I've had my 2010 for 10 years and no real change.

How long ago did you do the cabin filter and roughly how old was it? Thinking out loud you might have had some crap that got out when you did it.

 

x2 super weak, just pitiful…