Warriors do decent at PvP.
However, tell me this.
What spec are you? And what kind of weapons are you using atm? And what about the rest of your gear? Are they outdated greens? Up to date blues? etc.
Why I ask this, is a warrior is EXTREMELY dependant his/her equipment. If you have below average, your warrior will also perform below average.
Also, if you are pvping right now, at lvl 36, just the difference in levels can make a huge difference. Most lvl 36's will have moer difficulty fighting 39's.
In any case, here is what worked for me in the 39 bracket when I was playing my warrior.
First, GET YOUR WHIRLWIND AXE. This is attained through your lvl 30 warrior quest (which im sure youve attained since you were talking about intercept and zerker rage). Make this priority #1, as it will greatly help in lvling as well.
If you are a dual wield kinda guy, though I would not recommend it for PvP, theres two awesome swords you can attain through quests. One of the swords is "Sword of Omen". Search it up here to find out where the quest chain begins. Long story short you will have to clear out Scarlet Monastery. The second, is the "vanquishers sword". This can easily be picked up in the magic quarter of Undercity, jsut look around for it. You will have to clear out razorfen downs for this.
Second..Armor. If you intend on PvPing at the 39 bracket for a short while, I think the Scarlet armor is well worth it. This armor can be attained from the bosses in Scarlet Monastery - Armory or Cathedral. The Boots and the chest piece are very rare, so dont sweat trying to get these. For a chest peice, I recommend the "Green Iron Hauberk" - which a blacksmith can craft for you, or you can pick up in AH. On average I have seen them go for 10-15 gold on the auction house. The good part about the scarlet armor, is you would already have to go into the monastery to do yoru earlier quest, so youre kind of killing two birds with one stone here.
Third, GET TO 39. It will help alot!
Fourth...Your difficulty with casters is fairly common among most warriors. Once you have gear, the most IMPORTANT pce you can wear is your "insignia of the horde/alliance". This will dispel all movement impairing effects or other abilities that make you lose ctrl of your character (i.e. FEAR, frost nova, polymorph etc..). This will make a world of a difference against casters for you.
Fifth...For PvP, normally I would say arms spec is the best. Unfortunately, you dont have enough talent points for mortal strike. SO, i recommend speccing Fury, but using your whirlwind axe and spamming slam. Just wait and see how hard you will be hitting warlocks and mages for if you attain that weapon and some of the scarlet gear.
Here is a sample build that I think would work pretty good with a 2Hander in the fury tree. http://www.worldofwarcraft.com/info/classes/warrior/talents.html?000000000000000000000000550310100521205000000000000000000000000000
Key talents here are:
Creulty - Crits are a huge deal at lower lvl pvp
Cleave - If you are fighting more than one guy at a time, cleaving is a good idea if they are standing near eachother.
Piercing Howl - great for catching runners or slowing groups down all at once
Top 4..Imp intercept - This is an amazing talent for PvP. Talent it, USE it!.
Imp. Slam - This will be your bread and butter for damage. Use it whenever you can against opponents
Deathwish - YOu were saying you were having trouble against warlocks? Not anymore. Use this ability when one gets near you and slice him in half.
Flurry - Nothing better than have a big slow two hander, and have your swing speed increased. =)
Well, thats about all I can think of. Covered you gear all the way to talents, and where/how to get them. Other than that, its all how good you/your opponent plays.
Good luck