Friday I took my baby to the dealer for 30,000-mile service, for reasons, I do not understand exactly, but... I got scared they will void my warranty, and I listened to someone here saying "let them make some money, and they will take care of you later", and...

Anyway, apparently among other things they do, they rotated my tires. I told them to leave them as they are, since I got free rotation and balance, and the whole tire service from Discount Tire - they put new wheels on, they service them for life. Service department didn't listen, telling me it's my fault, because guy at the desk couldn't understand me. I have a slight accent, but I speak clearly and loud, so that was more offensive, than anything else.

I jumped on highway on my way home, and got very upset, as my baby was "ill-riding". The steering wheel started vibrating really bad once I hit 40 mph. It got a tad better after 60 mph, but it only stopped vibrating close to 80 mph. It was quite bad, felt like there is about half an inch loose in steering wheel, I had trouble to hold it.

It's Saturday, so dealer's service department was closed, and I didn’t have anyone to yell at. I took it back to DT instead, and they balanced my tires again, telling me the log nuts were not tighten either. Nice. I took it home. It was fine, but then I got very slight vibrations at about 55 mph. Of course it could've been tire guys that messed up this time, but they know my truck really well, and I never had problems with them.
So here is the question: what else could've been sc***ed up while servicing to make steerin wheel vibrate? I am mechanically challenged, so I am very careful with those things. And do they really flush all the fluids or only pretend to do it? And one more: I always thought that for tune-up spark plugs should be replaced, not just cleaned, or maybe I am wrong?
Any answer would more than helpful at this point, as I am a bit upset. Thx.

Kasia