Instead of steppers, have you considered using something like windshield wiper motors? They can be had fairly cheap from a junkyard or pick-ur-part place, they are generally worm driven (so no backdriving possible - so you don't have to keep re-adjusting the position because of wind or such - which I hope you were designing in for a stepper solution), and are fairly robust in the elements.
Even so - for either solution - you would probably want to house the motor in some kind of water/dust-proof enclosure, and only expose the shaft to the elements.
Another solution (so far untested by myself - though I am planning on playing around with the concept later this summer maybe) - drop the tracking entirely (yeah, I know, not as fun).
Find out what the average altitude angle for your latitude is to face the sun; this won't be the optimum angle for a panel, but really it will be close enough for most purposes throughout the year. Then all you need to deal with is the azimuth angle to track the sun (note, this could be done for a single-axis motor-controlled tracker as well).
What you do (again - this is untested) - get the lenticular screen from an old big-screen projection TV; people are throwing them away all the time, and usually the front lens is fine - maybe a few scratches on the lenticular lens (note - you'll also get a huge fresnel lens in the bargain - great for those times when you need to melt rocks or something of that nature!).
Place the lenticular lens smooth-side down on the solar panel, with the ridges "vertical" to the ground-plane. The "ridged" side is actually a series of cylindrical convex lenses designed to take the light from the projector, and spread it so that the image on the TV can be seen from a wide angle (150-170 degrees or so). Since you would be using it in "reverse" - it should be able to collect the light over that azimuth angle range, and direct it "straight" to the solar cells.
At least - that's the idea - like I said, I haven't tried it, but over the past couple of months I have managed to salvage 3 large fresnel and lenticular lenses from big-screen projection TVs that people were throwing away (bulk trash pickup in my area). The lenses are fairly easy to remove; I can strip one off a TV inside of 10 minutes. You'll also find a huge silver first-surface mirror inside as well (laser experimentation parts).
It's funny - most of the time when I see these TVs by the side of the road, people have stripped them of the projector, but leave the lens alone. Personally, I prefer the optical path components. Sometimes there are some nice mag-shielded speakers to be had, and if the unit was a triple CRT projector, and you are willing to take the time to carefully remove them, three large (4-6" diameter) projection lenses can be had too (note that some of these units used a coupling fluid between the CRT and the lens for heat removal - the fluid is something like mineral oil or such, and I am not sure how bad it is for the environment - or your skin/hands/body/health).