When everything else has been checked replace the center drag link ends (actually now when I hear compaints of loose steering this is where I look first):
http://www.gmsquarebody.com/forum/showthread.php?p=144531#post144531
That would be items 16, 17 & 19 in the dwg below. Any wear in this assembly will result in a loose steering wheel and road wander - no matter how tight the rest of the system is. If you can move either balljoint -it's played:
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Another little trick that is possible with a "center drag link" setup:
If your steering wheel is offset (either CW or CCW) while travelling straight ahead - it can be easily trued up by shortening or extending the effective length of the link.
Just glide to a stop (hands off the steering wheel). Your tires will be pointing straight ahead. Leave the steering wheel unlocked and loosen the clamps on the adjusting sleeve. Then with an assistant watching the steering wheel, turn the sleeve. The wheel will turn (either in the correct direction or not) if it gets worse, turn the sleeve the other way. It only takes about a 1/4 turn to move the wheel spokes about 45 degrees. This doesn't affect your front end alignment (both tires move in unison) only the index of the steering wheel. Much easier than adjusting both tie-rod ends!