I recommend that you check your local ordinances - there may be a law about this. Where I live, you may bury your deceased pet on your property, as long as you bury it at least three feet deep.
Depends on where you live.
Here I can dig a 4 foot hole with a post hole digger in less than a minute. You may need dynamite to go 6 inches.
I've learned that the deeper the better. I have foxes around here that will dig up anything dead I bury if its not at least 3 feet down. I've tried covering the hole with boards, blocks, etc.. only to have the fox dig in at an angle.
I bury 1.5 feet down about and cover the bodies with hot pepper powder- (hotter the better) hope to teach the local critters "food" here makes hot mouths.
Last March? As in March of '09??? And your vet still has them?? wow. Personally, I would go with cremation. I have no patience for digging holes. Sorry for your loss!!!
I buried mine down about two feet (couldn't dig any deeper) but then poured ammonia over the top of the dirt. I repeated the ammonia every few weeks and after nine months, nothing has disturbed the site.