To get to the original question:
"what is Aragorn's STR?"
in DnD terms I estimate he is at @14-16 myself.
Based on the fact that he is not overly buff, (in fact- with the insane carrying capacity of DnD Str stats a Str=18 charater would be pretty ripped, Conan- IMHO, but that is not important.)
and also the way he fights in combat.
He doesn't carry an over-abundance of equipment, and he usually fights with his sword in both hands; but does sometimes use two weapons, including his fist.
He must be getting enough of a bonus to make it worth while to use a second weapon or a two-handed grip, alternating between the two as the situation presents itself.
Also Gimli's weight is hard to pin but his regular armor must weigh less than 25lbs- because of the Chainshirt he is goofing around with before the battle, complaining there is not enough time to resize it. If he were wearing armor better than chain he wouldn't have worried about it.
Unless he was just messing around. He was pretty funny in this 'episode'. I would kind of guess he was wearing studded leather armor maybe? So either that or he was just goofing off.
Gimili may not be as heavy as he could seem.
In comparrison I would figure Boromir's strength to have been at @17; as he tends to use both a sword and shield.
Since he is not picking up a large bonus for using his sword two handed he decied to use a shield most of the time. IF he wear Str18 he would have used it 2 handed all the time to get his full evenly divided Str bonus!

He seems to be using a bastard sword and in the 'platinum' version -on one of the bonus discs- one of the guys (I believe from "WETA"(?) -who designed and forged the weapons) says that Boromir's sword is "a strong man's sword, it has a wide heavy blade and can be gripped with one or two hands." It was also perfectly balanced so I would say master crafted Bastard sword.
I would imagine Lurtz's Str to be @18? he was pretty buff, and of course all the uruk-hai are.
With regards to
tracking-
Well the Uruk-hai seem to wear leather armor, breast-plates, or banded armor. Mostly breastplate I think- I belive someone (Legolas durring the start of Helms Deep?) in the movie notes that
the uruk-hai wear heavy armor- he could only mean the actual army before them though,and not the war party from earlier.
So they would be moving at a decent pace.
Say they do wear breastplates but they have the
Run Feat and
Endurance Feat and it seems to be the regular orcs that get tired when they stop more than the Uruk-hai being tired and they only wear leather and studded leather.
Now the 3 heroes are trying to let Gimli keep up at about the same pace (Move 20) as the Uruk-hai and so it is hard to gain on them, At times Aragorn and Legolas run on ahead and Legolas can get sight of the group later.
As has been stated they would be on mostly
firm ground (DC15) and there are @15- or so- of them(medium sized), so that is a -5 to DC.
They are just under 24 hours away and there has been no rain and visibility is good. The Uruk are certainly not trying to hide their tracks. There is no game mechanic to show the will of Saruman and I don't think that anything shows he is tampering with the tracking.
Now if Aragorn put skill points to Wilderness lore at each level and is 6th level he would have a skill rating of [9+(Wis,say +2)]=
a rank of 11.
So we are looking at DC10 vs. a +11 rank bonus.
He could be moving at half normal speed off and on, they start out just several minutes after the Uruk-hai nab the hobbits yet end up being almost a day behind...
Also when they go to the battle sight I don't hink Aragorn is picturing the scenes in his minds-eye. It seems to me he is noting the tracks, and such, and making sense of what he sees and then we are being shown what Actually happened. He doesn't say "Pippen knelt here and cut his ropes on the fallen axe." He says "the bonds were cut". So anyhow I think he is seeing what the tracks are telling him, but on camera We are seeing how the scene actually played out.
Like the dagger sheath the orc grabs, I don't think Aragorn necesarily envisioned how the orc got it, I think "P.J." just showed Us.