While I was taking my front wheels off to work on the spark plugs, I grabbed the right-side wheel with it off the ground and was able to rock it in and out (hands at 9 and 3 o'clock, pushing and pulling) a bit. The left wheel didn't do that. Which expensive part do I need to replace?

 

You know mine does that too I think...I was thinking I needed an alignment

 

best bet is to have someone look underneath while you do that. i know the idler arms go bad alot on these things. could be a tie rod too

 

it could be your pitman arm, idler arm or inner/outter tie rods. you should remove the front skid plate and have someone move the wheel like you did and look at those parts, if you see movement then you have a worn part. but also grab the wheel at 12 and 6 o'clock and try moving it that way, if you can move it both ways (left/right and up/down then you have a loose wheel hub/bearing.

 

I did try the vertical rock as you suggested, and it wouldn't budge. Sounds like steering, then. The ball joint on the left was replaced just before I got it; I'm guessing the one on the right needs it, too. There's play in the steering, and I've been meaning to take it in and have it looked at anyway

 

yea, almost forgot, could be balljoints also.

 

I had the same problem before I lifted my blazer. It was the upper a frame bushings. If you look at them you can tell right away if its them or not. Also my ball joints needed to be replaced, once I did that I was able to get an alignment and the ride quality was back to normal. The ball joints were 15 bucks at autozone and the upper a frame bushings were 5 bucks each. I would say thats what your problem is

 

Ball joint would have play in and out holding it a 12 and 6. To check the ball joints you have to jack it up by the control arm to keep the suspension compressed. You can tell if they are bad if the suspension is hanging