the plumber came again yesterday and i was prepared for the worst. that anxiety was all for naught. sure, the laundry, shower, and kitchen pipes all lead to one pipe to the sewer line -- it's not the ideal plumbing set-up -- but at least we can take care of it if it backs up again.
what i was really worried about was the puddle of water under the house :: did we have a major leak? was the foundation being compromised? would we have to replace the piping? did our home inspector miss something major when we bought the house? the plumber's assistant crawled under the house and checked out the pipes and found nothing wrong, thankfully. and the puddle from two days ago was gone. the plumber's educated guess :: the water heater leaked water all the way under the house and it stopped once the water heater was replaced.
it still cost a lot to snake the pipe from the outside; double what the other pipe snaking guy charged us. it didn't bother me that much, though, as i did two loads of laundry and didn't once hear the gurgling in the shower drain, or smell the sewer in our bathroom. sometimes it's worth the price just to get the problem resolved ... after two years of suffering!