As others have said, I believe you have too much unloaded spacing. If the HDs are shorter, its even more important to use the spacers to get the clearance right. You want your unloaded truck to contact the Timbrens as little as possible, since you will feel every contact. Likewise you want the loaded truck to loose contact as little as possible, since you will feel every re-engagement.
I found the Timbren instructions less than helpful, recommending too little clearance. Once I removed the spacer, all was fine. So in my experience it takes a bit of experimenting to get it right.