Hi Bill,
Hopefully you've probably got it all sorted by now, but when I saw your message I have so much sympathy for you, as we too had the EXACT same problem with the house we moved into in Feb 2012!
Only a 50 foot X 20 foot rear garden, but when my wife merrily went out to plant a few sweet peas one sunny day and couldn't stick her fork more than 2 inches below the soil, we knew something wasn't quite right.
I soon dicovered that this was the case throughout the entire garden. Every square inch of soil was laced with all manner of builders rubbish up to a depth of 2 feet; broken soil pipes, broken cement slabs, iron hinges and nails, plastic guttering, met posts, bed springs, crushed metal dustbins(?!), bricks, you name it, it was in there.
I literally felt myself cracking up, having to spend our whole Summer breaking our backs digging out tonnes of rubble when we should have been sitting around enjoying our garden with a nice G&T in between watching the Olympics!
We had to hire a skip and take it all through the house in a wheel barrow there was so much of it and we've only now finished preparing the soil ready for some lovely new turf at last.
So you're not alone, it does seem to be a disturbingly common and tragic problem. Builders need to be watched like hawks so they don't do this to people.
Funny how the garden is never given a thought in a builders survey, yet a nicely done, pretty garden can practically sell a house...