How worried should I be?

Hey guys, this is my first time posting here, so forgive me if any of this is redundant. Also- thanks in advance for any help you can give me.
I just bought a 2002 Toyota Highlander V6 with 36,000 miles on it, and I love it. I decided to make the purchase after hearing/reading great things about this car, and of course, after getting it checked out by my mechanic, as well as reading the CARFAX and all that stuff.
2 days after purchasing the car, the check engine/VSC lights came on- I took it to the local Toyota dealership and they were unable to diagnose the problem (at the tune of $130, mind you). 2 days later the lights came back on, but went back off after I tightened the gas cap. The very next day (today) the lights are now back on, and tightening the gas cap is not working... I also felt what seemed like a hesitation this morning (like the back wheels grabbed or something) when I went to turn across an intersection.
I've come across a large number of similar issues within many messageboards suggesting that maybe the problem is my MAFvalve or my o2 sensor, but obviously, no one seems to have a for sure answer. I don't want to take the car back to the shop and spend more money! I guess basically, my question is this; As a first time Toyota owner, how much should I expect these kinds of problems? How serious are the problems I mentioned above? And ultimately, do these lights, and the other things ive mentioned, indicate to you that perhaps I've bought a lemon? How worried should I be about these lights?
Sorry if I am overreacting, but its just not comforting to be having these kinds of issues 1 week into owning the car.

First off, welcome to AF!

I would hope that your mechanic did a road test of the car as well as the normal inspection prior to purchase. They are keen at listening for strange noises such as low growling sounds, high-pitched whining, chirps, etc., as well as feeling the car for pulling, vibration, jerkiness, and difficulty in turning. If there is a problem with the rear wheel as you suggest, then return to him and ask him to try it again...see if he notices anything when you try to recreate the problem. He can also jack the rear up and spin the wheels to listen for problems with the wheel bearings and test them for looseness in the suspension.

As for the check engine light, it may be as simple as a faulty gas cap not able to hold the fuel tank pressure, either from the internal valve, a cracked o-ring, or a dented or rusted filler lip.

As for the useless dealership, he should also be able to read your codes AND TELL YOU WHAT THEY ARE. Some dealerships won't divulge what their diagnostic scanner reveals. They only tell you what they think is wrong. Go back and insist on the actual trouble code(s) that are causing the check engine light...printed out. It doesn't come on for no reason. The point is, you paid for the service, you should get something out of it...the diagnostic report, codes stored in the car's computer, ALL OF IT. If they are unwilling or unable to give that to you, ask for a full refund and move on. A nice hint of a complaint to the BBB (or equivalent) and the town's business regulation authority can sometimes help loosen their grip on your money.

Good luck to you and welcome again!

Thanks for your reply, man. The back tire gripping happened 2 days ago, and hasnt happened since. Its quite possible that was more the result of my driving than something else. Still getting used to the car, you know? If it happens again, Ill be sure to let you know. As far as the CEL/VSC lights go, after unplugging the battery 2 days ago (I read that I could do that on here), they havent come back on. Hopefully it is just a faulty gas cap or something along those lines. And yes, when I took the car to my mechanic, he did drive it. We rode around for about 20 minutes in the car together, and like me, he thought it drove really nicely. He also did everything else you mentioned, pretty much. The bottom line is, is I got a reallly good deal on the car, and these lights have made me suspicious even though I think Ive done all the necessary research. Its even more overwhelming that so many people seem to have issues with these lights going on and off all the time, and are unable to diagnose the problem. I'm just a little wary bc I have bought a lemon before, and really feel like I did everything in my power to prevent that from happening this time. Although none of the most common issues Im finding relating to these lights seem to be too serious, Im just a little nervous. If the lights do come back on, I will definitely take it to AutoZone or somewhere where they can get the codes, and I will put them up here before I take it back to the dealership. Thanks again for your response, and for your time. I will keep you posted.