Thanks. Since I'm getting cooling from the AC at all times, I'm guessing it is not the clutch. Linear decrease is close to accurate -- perhaps a little closer description is that I have really cold AC for the first two hours or so, then a rapid dropoff to a plateau of mediocre but steady cooling. Basically, it falls off to mediocre cooling, and stays there -- it doesn't degrade any further.
I will clean off the condenser too -- good idea, can't hurt.
I pulled the cabin air filters out this morning, and vacuumed what I could out of where they go. They weren't too bad, not that much crap up inside there. I ran the blower motor with it all open to try to blow out any debris -- only a little came out. Pulled the engine air filter too, and removed a large mouse nest from the filter box.
I swapped the AC relay with another relay in the fuse box.
When turning the blower motor on/off, a bunch of water drained out of the evap drain. Is this normal -- for the morning after I drove the truck for several hours? I did not drive it at all this morning, so all that water is from yesterday's condensation. If the condensate was freezing up or blocking up somewhere, maybe that's what's causing my AC degradation after a couple hours of driving.