Max, I am building a four link with threaded adjusters. I used grade 9, 3/4-16 bolts, they are only adjustable on one end. Do I still need to run lock nuts to keep everything tight?
You always run jam nuts to tighten up any play in the threads. A little bit of back and forth movement from the clearances in the threads will get worse and worse over time and eventually cause problems.
We had a customer with an SD 5 link who stripped all the threads off one of his adjustable ends because he was driving with a loose jam nut. Nothing was holding the bar together it was just a sliding fit where the threads should have been. Always run jam nuts and check them regularly.