i tend to agree with 'Rich' comments.
While container shipping/sharing is a low cost way to shift the vehicle have you considered the time it takes to arrange paperwork and loading of container, there's also the potential of ship arriving late (or leaving early!) not to mention the time lost during the movement on water.
It's all perfectly logical for a long trip but if you only have 4 weeks to spare in US then I think it is not the most convenient (and probably not cheapest) option.
we have shipped in a container a few times with our 4x4. Every time it has taken a chunk of time to ensure all paperwork is correct and then you need to allow a further day or so for liaising with port and getting container actually loaded.
I would suggest as a ball park figure allow 3 work days each end minimum for loading and allow 2 work days for unloading.
Often people who have already gone through the process of shipping will be trying to sell their vehicles rather than ship back.
My British reg. 4x4 iveco will be for sale in / around Argentina next April and though no fixed price decided yet I can assure you it will be less than the cost of a container.