Here's a weird one - I had this same issue on my 62 where it wouldn't lock up after taking out my instrument panel and fixing the speedo, but then not re-engaging the speedo cable completely into the back of the speedometer. Truck drove totally normal and good until I got up to 51ish on the fwy, then it wouldn't lock up. The speedo needle was dancing more than normal and was making some racket. I pulled over, pulled the instrument panel out (lucky it's not hard on a 62), and cut up my hand re-engaging the speedo cable, and it was all fine after that.
I know the lock-up is a mechanical thing, but best I could figure there is some kind of speed sensor that the trans needs to engage the lock-up.
Hadn't been mentioned, not sure it helps, hoping it could be this simple... all the things. You'll get it. Good luck!