If you're literally shaking your whole house, you're certainly not doing your house any good. This is a structural issue and should have been addressed when it was being built. I can't imagine going years like this either. I would have at least gotten a different machine or do something with it immediately. You could try putting down a couple 1" sheets of plywood and screw these into the floor joists securely, and use a lot of screws to make it solid and see if that helps. There's anti-vibration mats but I have no idea if they work or not. Your issue sounds like a poorly constructed house. Since it's a structural issue, you're going to have to address it structurally. Maybe best to get a competent contractor to come out and give you an estimate on shoring it up. Whether adding blocking to the floor joists, topping with plywood or whatever, you can at least get an idea what it'll take to get some stability in the place.

It can also help to have them near a solid, load-bearing wall since it'll be more solid there. You might try moving them. The problem is that there's no guarantee that you won't have troubles. There's so many factors that weigh into the equation and 2nd floor installations are finicky.

If you don't want to look at structural reinforcement your only other option is to look at different machines. There's a lot of threads here about second floor laundry installations and the problems inherent to the design.

One thing I'm pretty confident of, that is you won't have an easy solution, nor will it be only a few dollars.

What kind of machine did you have there before? I assume it didn't shake the house when you had it for 3 months so that should give you an idea of what type of machine would be more compatible. I assume your problem is with the washer only? At least you wouldn't have to replace the dryer unless you're one who must have things "match". There's HE top loaders that would be an option. There's a couple front loaders with a better suspension who some claim are better for second floor installs.

One other thought, if they're on pedestals, you could try taking them off. Some report increased vibration issues with pedestals.

Try these vibration reducing pads in the link. I have not tried them but they look like they may help.

Here is a link that might be useful: Shake Away Pads

We looked at a home that was for sale, several years ago, and the owner actually had the home built with a concrete pad where the washer sat in the ground floor laundry room because he couldn't stand the vibration of his washer. This cost him thousands of dollars - and it was his big selling point about his otherwise beautiful home.

That's when I told the home owner about our then newly purchased a Fisher & Paykel GWL 10 Ecosmart washer (2001), which you can't feel spinning at all unless you place your hand on the machine. No vibration on the floor at all. A washer that doesn't "walk" around on it's own. I suggested if he purchased a F/P when he moved, and gave him the name of the local store that carried and serviced them, he could avoid all the expense of putting in another concrete pad in his laundry room.

The Fisher & Paykel also has increased reliability with no transmission, belts, brakes, clutches or pulleys to fail, let alone NOT shaking your home to bits - as a nice side benefit.

-Grainlady

You might check this thread out:
http://ths.gardenweb.com/forums/load/laundry/msg010912587741.html?12

I would try reinforcing your floor like that first before going to more expense.
It's working great for me!

I tried the KE Shake Away Pads like the user above. They barely worked.
I had the same shaking problem and someone on a different post had mentioned Silent Feet. WOW. These pads are amazing. 95% of all shaking and vibration is all gone. Get these. Don't just take my work. Read the reviews. I bought them on Amazon.com. Best $40 I've spent

Here is a link that might be useful: Silent Feet website