i have a 79 K5 full size, i recently went from 35x14x15 rims and tires to 35x12.50x16 and i had the ball joints replaced and a allignment dont, not i get all kinds of road wander, feels like the rearend is trying to pass the frontend, i can hardley stay in my own lane when accelerating or letting off the gas. rechecked all bolts and everything is tight, need some help on what it might be.

 

Those K5's are pretty heavy , i think. It might be that they put on tires that are unable to carry that weight.
also, you went from a higher, thinner tire, to a smaller , wider tire. Try putting more pressure in your rear tires. Did they balance your tires and are you sure they are put on straight? Try taking them of and let it roll on a flat surface. See how it behaves. The tires should go straight on. (I know this isnt the most effective way of testing tire balance, but it gives you some insight...). Put them back on and fasten them crosswise.

It could be a number of things of course, but maybe you should try to find a pair of your old tires back. And see if the car still behaves that way.

 

What kind of tires are they Mike? Are they new or are they used?
If they are used radials and are mounted differently than they were on the last vehicle that could be it, also the tread may be worn abnormally causing the problem.

 

they are new micky thompson MTZ 35x12.50.16 . found out that my tie rod's were bad, replaced them along woith the steering box. but now it is still doing it. the tires are 8ply they cal for 50 lbs cold max. the tire shop re balanced the tires , and the shop re checked the allignment,and with new bushings and new ball joints and newsteering box, and new tire rods, i am at a loss of what is going on.

 

well it was the wheel barrings. worn down the axel shaft some... fixed now after going threw 4 diffrent axels to get 2 good shafts... and a couple days worth of work in the sun.