I would think that chemicals would hurt anything that breathes.

Try drenching your garden to wash off all the chemical. But it may be too late.

There is much anecdotal information, supposition, and speculation about this but I cannot find any real research that says so. I do know that lawns I have worked on where weed killers have been used regularly have fewer earthworms that look unhealthy then lawns with adequate levels of organic matter that weed killers have not been used on.

i have always thought that Round Up killed Earthworms.

It is well known glyphosate products do have an adverse affect on many forms of life where they are used causing deformity, slowing growth, inhibiting reproduction, and populations of them decrease when those productgs are used. But many studies indicate that earthworms can live, not well, in glyphosate treated soils unless the amount is considerably above what would normally be used, according to what I have read.

Yes, it could be the Ortho that's making the worms sick. I would try to remove the top layer of the soil where the contaminated mulch is. Not only will Ortho residue kill the worms, it will throw off the natural balance of the soil and adversely affect the plants over time. So will Roundup and snail and slug poison.

It is hardly necessary to remove the top layer of soil as long as some form of organic matter is added. I can find nothing that indicates the synthetic plant poisons outright kill members of the Soil Food Web, except supposition such as that above, but there is evidence that they can cause genetic mutation in lower life forms where used as well as possibly make them ill.
Most all of the synthetic insect poisons will kill off members of the Soil Food Web, just as they will kill the unwanted and wanted insects.

You should try killing off dandelions the better way, that works, instead of wasting your time, money, and effort while poisoning your soil.

Here is a link that might be useful: http://www.richsoil.com/lawn-care.jsp

The current issue of Readers Digest has an article that outlines the problems with using the toxic, synthetic poisons sold in all of the garden centers and used by many people. Unfortunately there are too many people that will not believe that any more then the information that has beed around for many years that shows we are polluting our world and poisoning ourselves.

Meh. The concentration from one application of broadleaf weed killer is thin. Then you mowed SOME of that into your bag, and spread it all over your soil in your garden, thinning the concentration further.

I'd be more worried about some of the weed killer transferring to my garden plants, but even that is a slight chance, since it is absorbed by leaf-surface contact, not by roots. So unless you put the grass clippings ON your plants ...

Doesn't explain why your worms seem sick, but I doubt it was the weed killer from your lawn.