I think most Paladins get up that because of Holy shield. That skill is usually maxed in alot of builds. I have about 1.6k def before HS, it then climbs to about 16k thanks to HS and a Defiance merc. Maybe you could also post your gear as to why your def is low.
15k defense is not that hard at all to get with a pally. My pure foher, which is a caster build has 18k. Smiters and zealers usually have upwards of 40-50k defense, all this done by holy shield and its synnergy defiance. You also have to have some high defense items.
40k+ is generally only achievable if using an ethereal-bugged Exile. A good
Exile Vortex can have 1600+ def on the shield alone.
Couple that with a 1600+ def Fortitude Archon, whatever Helm/Gloves/Boots/Belt they're using, the Auto-Defiance aura on Exile, Chilling Armor proc from Fortitude and Holy Shield, and you'll start to see those super high numbers.
My Zealot has 16k with just Holy Shield, and 20k when Chilling Armor from Fort procs. I don't use Exile though, just an upped HoZ.
oh, and GreyGhost, the Auto-Defiance on Exile wont "synergise" with Holy Shield per se, it'll just be an aura running on top of Holy Shield. A decent Zealot/Smiter using Exile can have a total of +900% defense or more with a fully synergised Holy Shield.
i use an 900def coh and regular gores, andy's helm, dracs, bk, ravenfrost, and highlords.
I have 22k def with an exile shield that is under 1k and has a lvl 13 i think defiance. That is with a level 30ish holy shield. with a bugged coh, its 24k i think.
I heard of some guy that had 70k def. Im assuming he had all eth gear and eth bugged runewords in order to reach that. nobody could hit him unless it was smite or some other "always hits" type of attack.