Did you remove or replace the center link?

The center link can easily be installed "upside down" and fit, and look correct. I had a friend who had the same issue on his 76 after doing a front end and suspension rebuild.
The alignment tech missed it the 4 times he was in to get the truck to drive right, it wasn't until the shop owner (an older gentleman with years of service work under his belt) went out to see what was going on. He immediately spotted the incorrectly installed center link, and after a quick flip and alignment the truck drove perfect.