Are the tires inflated to proper specifications? Do you have traction control on your Odyssey, and, if you do, is it switched on or off?

Have you ever had the tires rotated during that two year period, and how's the wheel alignment? A lot of wear can occur in a short time/mileage interval if the alignment is out.

Thanks steve and exgm,

Yes, they are properly inflated, and yes, the TCS is always on.
But....I just thought of something. I take it in every 3,500 miles for its servicing. About 4 services ago, the manager told me that one of my tire rims was dented, and that meant they couldn't rotate my tires (only if I would buy a new rim). I decided to forego the rotation. 2 services ago, they rotated them and said there wasn't a problem. Well.....I noticed a shimmying of my steering wheel after that and told them, and they put the tires back to the position they were before the last rotation, and the shimmying stopped.
So......I guess maybe that's the reason for the poor traction?? Do I have to buy 4 new tires and a new rim to get better traction, or could I just buy one new rim, and then have them rotate them?
But......they told me that even though they put the tires back to their pre-dented-rim location, they couldn't see a dent in any rim. This is a very reliable service department, so I don't feel that I'm getting the run-around. But I am a bit confused now. Any suggestions?

You don't have to rotate tires. You can, if you want, leave them in place, and replace the front pair, or the rear pair, independently. Front wheel drive vehicles will often wear out the front tires much faster than the rear set, and so the front pair would be replaced sooner. A bent rim could cause shimmy, but worn suspension components could also be the cause. Maybe one of your tire/rim units is out of balance, & causes the front end to shimmy. In that case, having the wheels balanced might solve the problem. If you have considerable tire tread remaining, and yet you have poor traction, could it be that you are using too much power?

There are two trains of thought on rotation. I don't recommend it on properly aligned rear wheel drive cars because their wear was pretty much equalized. For front drive cars, the tires on the front generally wear faster. Also, rubber ages, and therefore traction capability also changes over time. If you rotate all tires, and wear them equally, you replace all of them at the same time and have a relatively balanced amount of traction from them all, again more important for front dive cars.
I have found that even though visually the tread appears to be fine, there is a definite wear pattern to it, and tire rotation usually means you end up with more rubber in contact with the road. I've noticed it particularly with all season tires. It would be the first thing to try, but in your case with the dented rim, I'd want to be sure that was addressed first, if only one is dented, I'd replace it. Steel rims are generally inexpensive. Regardless, have a pro look at that aspect rather than taking my word for it, remember the tires are all that's keeping you on the road!

Thanks eric and exgm.

stuck in your yard? like dirt or mud? do you do a lot of off roading? i have a hard time seeing how any FWD vehicle with fairly good tires would exhibit wheelspin on a paved wet surface. which Michelin tires are they? symmetry? they are so-so as far as traction. do you live in the country? I am not picking on you, just trying to figure out your situation. a wet paved surface is not slippery. a muddy ditch is very slippery.

TCS always on...

That could be your problem. With TCS on the instant wheel slippage occurs the brakes are applied and the engine is dethrottled. Turn TCS off and you will be able to "rock" the car back and forth to get unstuck.

If there is no TCS off switch, many FWD vehicles do not have due to the inordinate danger of operating a FWD absent TCS, there is a simple procedure that can be used to temporarily disable TCS and VSC.

While the engine is running unplug the elctrical connection to the MAF/IAT module. The engine will die, then plug the module back in, restart the engine. You will now have a CEL, TCS and VSC diagnostic which will reset within a few drive cycles.

In the meantime TCS & VSC will be disabled.

You undoubtedly have either an all season or summer tread tire. When in very soft conditions, you will require a mud tread or all terrain tread with a high void to rubber ratio(that is a large spacing of the tread lugs). I have experienced a similar problem with my Michelins on Dodge 4wd pickup. I solve the problem for me by engaging 4wd to get back onto my driveway. I chose the tires on purpose for maximum mpg and longevity.