Since you said it's a straight choice of SK or Ranger, I'm not going to suggest anything else.

To be honest, either works well with a shaman. I won't recommend one over the other, but some strengths and weaknesses. I've duoed a lot with rangers in the past, and they're not great tanks, but they're not bad either. They need decent gear to tank well, but they have good agro and good dps. For average content, what they lack in tanking, they make up for in killing stuff fast. At lower levels, rangers have some outdoor pulling tools too, which can make things easier. A ranger also has nice buffs that complement shaman buffs well. At later levels, the ranger gets archery AAs that can make him a decent box class to stick on autofire and forget.
However, an SK is a better tank. There will be things that you can tank with an SK that you can't tank with a ranger of similar gear. An SK also has split pulling abilities which can be very helpful.
Also, if you're planning on grouping up at some point, an SK/shaman duo is probably a better basis for a group - you already have 2 of the key roles there (out of slower/healer/tank). In terms of finding 4 pickup people to fill out the group, it's typically easier to find a dps /lfg than a tank.
In terms of your 2 boxing 4 toon group, there are lots of viable combinations. I would suggest adding a cleric because then you always have the best healing, buffs and a rez, and you never have to look for a cleric to do certain things. Though it will slow down killing on other things where shaman healing is sufficient. Also, for the 2 boxers, casters are easier to box than melees, because you don't need to worry as much about positioning.
Here's a few suggestions:
SK/shaman/cleric/wizard - best slow, good tank, good buffs, best healing, best rez, ok dps, evac
SK/shaman/druid/necro - best slow, good tank, ok buffs, good healing, rez, good dps, evac
Ranger/shaman/paladin/wizard - best slow, good tank, ok buffs, ok healing, rez, good dps, evac
Things like evac/port and rez aren't essential by any means, but they do make your group more self sufficient. Rez means fewer summoning and asking around in Guild Lobby. Ports still make travel quicker (and cheaper) and evac means less recovery time if things go bad.
There are lots of other combinations that would work well, but maybe I've helped you think about what you want from the group a bit too.