Yes, diagnosing is probably going to be the answer. I have not been the best at diagnosis and I had "starter" trouble and changed my starter, ate up my flex plate and spent all kinds of time chasing the problem. I ended up putting the old starter back on. I'm not saying you have the same problem I had, but the problem could be with several different things. In my case it was bad battery cables. I don't know if we've seen pictures of this part of your truck, but if you have this type of emergency repair terminal, change out your battery cables and do not use this type of terminal. Too much and embarrassing info in this other thread, but lesson learned:
https://67-72chevytrucks.com/vboard/showthread.php?t=734496
I really dislike those emergency repair terminals. Don't know if you have them, but if you do, I hate yours, too.