This build is derived from the common 11 Barbarian 8 Druid 1 Favored Soul, and plays almost identically when specced for DPS. The build I am making is the following:
12 Barbarian 8 Druid, True Neutral Dwarf
The goal of this build is to be quite sturdy and have reasonable amounts of DPS at any point, with the option to swap enhancements and gear to become incredibly durable. Also in the words of Strimtom, TAKE THE GOOD STUFF AND NOT THE BAD STUFF!
This will not be a complete build guide, though I may be adding one later as I level and refine this build.
Edit: I think this build does great on elite and low-reapers, and with proper gearing mid-reapers should be possible to tank. In high-reapers, this build definitely lacks compared to true tanks such as paladins or fighters or pure barbarian even. You have several abilities that serve you benefits (like debuffs on them and buffs on you) when a monster is swinging at you, and more if
So, you are probably wondering "what do we gain with that extra level of Barbarian?" Well, we get access to the 4th Cores of both Occult Slayer and Ravager when we swap to be a Tank. I know there are numerous builds on the forums, so I will only detail the enhancements for the Tank build. The 4th Cores of Ravager and Occult Slayer alone give us 120 HP, 5 MRR, 40 HAmp, Slippery Mind, 1% Barb damage reduction.
With 80 AP, we will put 34 AP into natures Protector, 24 AP into Occult Slayer and 21 AP in Ravager with one leftover. If you have Racial AP you can use this point for either extra HP or to access the T4 in Dwarf for a total of 25% more strikethrough and +4 to Hit/damage with your Dwarven War-axes when tanking with a shield.
In Nature's Protector, we will grab as many defensive abilities as we can, including a Defensive Stance and Cores 1-3, Threat, PRR and MRR, Saves, and AC. We will take the Tier 5 enhancments, with 3 Ranks in Force of Nature and Beast Unleashed. This will also let us use Metal Armor for more options for Gear Sets.
In Ravager, we will pick up Cores 1-4, HP and HAmp, passive Debuffs for enemies, one source of temporary HP, and CON while raging.
In Occult Slayer, we will pick up Cores 1-4, a second source of source of Temporary HP, CC against casters, CON and HAmp. In Tier 4 we pick up Driving Force for 20 MRR with an 83% uptime. If you have trouble with short-term buffs, swap this for the 9 MRR from Ursa's Will.
There is at least one downside to this split: you miss out on Magic Missile immunity from Nightshield, and relies on either clickies or using wands out of Animal form. For any build without innate Magic Missile Immunity, both the heroic and epic Sapphire Studded Buckles is an awesome item for both shield clickies and damage in the early levels, and the Epic version casts at CL 20, meaning you have a full hour of Shield.
As a tank you will want to get as much Intimidate as possible like any other tank, but your increased hate and reasonable damage allows you to keep agro on some constructs, some undead, and vermin through the hate mechanic (which cannot be intimidated).
One last thing I will say: PLEASE PLEASE PLEASE slot Treachery for reduced threat if you are raiding on a DPS toon. You do NOT want any defensive stances that will add threat: someone like Suulomades will turn and 2-shot almost everyone in the raid.
I am open to any suggestions! The one requirement is being able to swap from DPS-focused to Tank-focused with only swapping enhancements and gear.
Edit: This build does great, and with proper gearing low reapers and even some mid-reapers (with a group maybe) feel real smooth. However, in high reapers, this build falls behind other true tanks for two reasons, and the difference is substancial. Several abilities you take from the Barbarian trees cause debuffs to enemies and buffs to yourself, as well as the Druid spell Rising Fury. This makes it beneficial to be exchanging blows with mobs rather than kiting or simply avoiding the incoming attacks, though if you have enough Intimidate this is still possible. Though if the reader is playing in high reapers, I assume you have the experience to know this and either avoid this build or gear/build around these issues.