Well, since you have a WHM, I would suggest you go for the GLD/PLD route first. Since you already have the cross skills from CNJ available to him. Granted, these cross skills don't give the best utility, but cure at early levels can help sustain you in solo and stoneskin and protect will help a lot with solo leveling too. The GLD is the basis for all tanks imo. We contain Provoke and Convalescence; two very important tanking skills. Flash is also very helpful as it allows for another way to gain enmity if you run out of TP with MRD. Not quite helpful to a DRK as they use MP anyways (so it doesn't really save you anything). Only reason to use flash with DRK would be if you DON'T want to hurt enemies while gaining aggro (maybe you know your magic user's will sleep some enemies). But that's situational.

Pending what you are looking to do with your tank, I would say GLD/PLD will meet all your needs. However, you'll have to level MRD regardless for the cross skills there (well... to gain the most out of your tank). Foresight is nice as it gives more defense (and for a tank, always a good thing). Bloodbath is pretty nice, but it's not nearly as effective as with the other tanks since you have no means to mass hit enemies (therefore, it's a slow increase to your hp). Usually, it just negates your enemies auto attacks on you. You won't really "gain" hp with it as a PLD. More like you'll stay stagnant in your hp if you're fighting something at the same lvl. Mercy stroke is helpful early and for solo playing. Later in game, it can be helpful... but getting the timing right on the kill shot can be difficult. But it's also an off GCD attack, which is nice too.

I wouldn't suggest going DRK first as it's cross class skills are both GLD and MRD. My personal opinion; go GLD first, then MRD, then DRK. Get GLD to lvl 22 for Provoke. Awareness at lvl 34 won't be needed unless you get a high lvl WAR as it negates the lvl 56 WAR move raw intuition's weakness. After you get to lvl 22 and get provoke, switch to a MRD and lvl to 26. You will gain every cross class skill a MRD can give at this lvl. At this point, the choice is your's. Did you really like playing GLD or MRD better? Or, if you're really curious, you now have all the needed cross class skills to move to a DRK and test it out. From there, it's really all about play style preference. As you are a healer, I feel you'd probably like PLD or DRK more. PLD contains more support type moves (but I will also point out, many are situational and hard to set off perfectly). DRK is a mana user, so it may help you in that you usually pay attention to your mana bar. It still dishes out good dmg too. However, if you are totally looking to change things up, WAR will probably be the avenue you'll wanna take.