But the problem is that at the higher levels, there is no real point to having a high AC and a high DR since most attacks will just cirumvent those things.
Nothing circumvents DR X/--, and DR ~10/-- combined with Elusive Target is a pretty damn good combination for any kind of "tanking" character.
As for how to change armors to make them useful, let me pull some homebrew out of my ass:
1) Cost of Armor Enhancements is doubled, so +1 armor costs 2k, +2 armor costs 8k, etc. Look at the weapon table to see what I mean. On the other hand, the enhancement bonus now applies to DR provided by the armor. Shield costs are unchanged.
2) Special Armor Abilities with a cost based on an enhancement bonus have the cost of the bonus reduced by 1, to a minimum of 1, so Heavy Fort is a +4 instead of a +5. As such, an old +1 Heavy Fort suit of armor costs 36k while a new +1 Heavy Fort suit of armor costs 32k. Abilities put on Shields are, again, unchanged.
3) Light armors get the following overhauls:
Studded Leather- DR 1/--
Chain Shirt- DR 1/--
Even though they provide no DR themselves, something like a +1 Leather Armor will provide DR 1/-- as normal.
3) Medium armors get the following overhauls:
Hide- +5 AC, DR 2/--, Max Dex +4, ACP -3
Hide is the super-budget medium armor, and the only medium non-metallic armor.
Scale- +6 AC, DR 3/--, Max Dex +3, ACP -3
Budget medium armor for 1st-level characters.
Chain- +7 AC, DR 4/--, Max Dex +2, ACP -4, 200 GP cost
Low-dex medium armor for experienced characters.
Breastplate- +6 AC, DR 3/--, Max Dex +4, ACP -3, 250 GP cost
High-dex medium armor for experienced characters.
4) Heavy armors get the following overhauls:
Splint Mail- +8 AC, DR 4/--, Max Dex +0, ACP -6, 100gp cost
Budget heavy armor for 1st-level characters. Let the Fighters and Clerics put that proficiency to use at first level.
Banded Mail- +10 AC, DR 4/--, Max Dex +2, ACP -5, 400gp cost
"High"-Dex heavy armor for experienced characters.
Half-Plate- +11 AC, DR 5/--, Max Dex +0, ACP -6, 500gp cost
Low-dex heavy armor for experienced characters.
Full-Plate- +10 AC, DR 5/--, Max Dex +1, ACP -5, 600gp cost
Middle ground between Banded and Half-Plate.