Well.. seeing as you say your original question was more about end-game tanking, I'll answer your new question with that in mind (so it might not all apply to you yet).
For a straight up pull with no frills, I would throw my shield, drop a Consecrate, judge Crusader (unless there is a Ret to do that, then I would judge Wisdom), Holy Shield (you can Judge & Shield at the same time since only Shield triggers the GCD), seal Righteousness. I would have Ret Aura up for this.
One of the important skills that you will develop is when to alter this. I often skip Crusader on trash and hit them immediatly with Righteousness so that the DPS can open up. I'll throw an Exorcism in AFTER judging Crusader if the mob is an undead/demon. I might not Holy Shield on all pulls.
There are a couple somewhat universals though. I will ALWAYS have Crusader on a boss. If there isn't a Ret to judge Imp. Crusader, I will judge my base one. This is even true when I vastly outgear something and know I won't be taking much damage. Wisdom is a second priority on bosses. On trash, I will judge based on my discretion and what I feel will benefit us more. I have never run into a serious DPS mana issue that hasn't been resolved by having a Class Leader help out the people having trouble and making sure everyone's on the same page.
The other semi-universal is that Ret Aura will usually be up. It is important to know when to change this. When I DO have to change it, it's normally to Concentration Aura (untalented) and occasionally to Devotion Aura. Every situation can call for something different, and you need to know which tools to use to handle them.
As for your tactics. I wouldn't switch targets at all. Your reactive and AoE damage should generate plenty of threat on secondary targets. You need your focussed abilities to keep control of the main target though. Just make sure everyone follows a kill order or /assist you after kills.
Righteous Defense is strange. It used to be that you had to select the person who was being attacked if you wanted to use it unmacroed. EVERYONE had a Macro that let you cast it on the mob that you want to taunt though. Since 2.4, you can now cast Righteous Defense on the mob that you wish to taunt and it will automatically cast through them and on the guy who's under attack. Righteous Defense works on the melee hit system, so it can be resisted. This will happen more frequently if the mob is a higher level than you. The person that you want to taunt the mob off of also has to be in range. The mob might be 2 yards from you, but if the person ran ape@#(* in the other direction to get away, he might have moved out of range. Also, not all mobs are tauntable. Many end-game bosses aren't, and even some trash.
Seal of Vengeance is an Alliance Paladin's seal. With the right weapon, it can generate a greater amount of threat over a period of time than Seal of Righteousness. It is not entirely reliable, and the stacks that proc from it could fall off of the target. On the whole, Horde pallies don't lose much by not having it. Seal of Blood is the Blood Knight unique seal. It causes damage to you when you do damage with it. This has a couple of benefits. It means that Ret pallies will NEED healing when they aren't being hit. This will return some mana to them. It also means that a Protection Paladin who VASTLY outgears a particular fight can swap Righteousness on their action bar for Blood and they will put out more threat and need more healing, which will return more mana to them.