I'd agree that the center arm up front usually causes alot of wandering. Just have someone sit in it and rock the wheel back and forth moderately while you look around at all of the joints until you find where the slop is.
The quickest and easiest thing to check is the center arm as the other guys said. To check the adjustment, loosen the small bolt at the top of the assembly, then tighten the big nut under it down until it stops. You don't have to wrench down on it. Then back the big nut off 1/2 turn and tighten the small bolt while holding the big nut so it doesn't turn. Make sure you get the small bolt good and tight so it doesn't loosen up by itself.
If you had to turn the big nut more than 1/2 turn, then this should have tightened things up some. It may not be the only source of "slop" but it will help.
After that, you will probably need to replace the tie rod ends. To seperate the tie rod end from the center arm and the steering arms, use a "pickle fork" tie rod end seperator. After that, they aren't hard to replace, but you may need to put the ends in a vice after removing them from the truck to get them to unscrew. Keep in mind tht a couple of them are reverse threaded, so they unscrew in the opposite direction. (I don't mean to insult your intelligence, but I don't know how familiar you are with these things. I had never done anything like this before, so I needed all the help I could get.)
I just went through this with my truck several months ago and ended up replacing the TREs, the rag joint (which connects the steering wheel shaft to the gear box on later model FJ40s) and rebuilding the center arm and the drag link end which connects the drag link to the pitman arm. I also went ahead and installed the factory power steering gearbox and pump that I bought used.
After several (dumb) minor mistakes, my steering has about 1/2 to 1" of play in it. It's a lot more fun to drive now and I can't believe how bad it used to be and that I could even drive it like it was...
have a local shop check toe in on the front end, mine liked to follow ruts and cracks even after rebuilding the front end. i took it to the local tire place (les swab) it was only off by 1.62 deg. and should be set at zero acording to them. after it was done it drives perfect even at high speed on the freway and they did it for free, probably because thats were i got the tires (35's) a while back. the adjustment normally costs $30.00, good luck!
Check TRE Condition, Center Arm Condition, Joint at the Steering Stabilizer etc, etc. Just finidhed Replacing TREs and Stabilizer and can notice the difference at >25mph oh and word of advice PAY ATTENTION TO WHAT YOU'RE DOING...mine is a lil off cause I screwed up but can work for now so I'm not gonna touch it for now. Also make sure to buy QUALITY parts if you do start redoing TREs.