1 question 1 problem with ball joints

First ill start with the question. For the past couple months my service engine light had begin to turn on. However once i get gas it seems to turn back off. Well tonight i was driving home and once again it turned on again, except this time the tank was only 1/2 empty (or full: depends on your outlook on life :) Anyway has anyone else experience something similar and might know what it could be.

In addition im going to be replacing a HUB on the front left side along with all the ball joints. How bad of a job is it. The reason im replacing the hub is because the ABS is going off at slow speeds. I read about cleaning it but i think the bearing is going as well. I have heard doing the ball joints are a bitch. Is there any way to check if they are really going bad before i start?

-- 98 Blazer 4x4 with like 141,000 miles

go to autozone and have them get the codes for you.

uppers aren't to bad to do. i did both my self but the lowers i had done. i heard they are the harder ones to do.