Beleive what the mechanics are telling you. The clacking noise is really comming from the rear drums. I had exactly the same noise when I replaced the drums (and everything inside) in my ’99 van. Also, because the new aftermarket drums where not balanced correctly, I experienced a lot of vibration in the chassis with the worst @ 55MPH+. The odd of all was that the vibration was felt in the front of the van.
To solve it, I put the original drums back after having them turned. I also used the best aftermarket brake shoes I could found. Sometimes, not always, I hear the clacking noise but it is very low and soft that really does not bother me at all because I have to pay a lot of attention to hear it.
I hope it helps.