Mystery vibration at 25 mph can't be diagnosed. It's not the front end!

The rear end has been that high since I bought it. That was 4 years ago. The previous owner put truck(?) or station wagon(?) leaf springs on it because he said that he couldn't find new ones for the dart. Well, that was the next thing I was going to correct. I found the correct 5 leaf springs from classicindustries. I have had to chase down a ton of parts that were just slapped on it for a quick fix. That is just one example. The dart didn't start shuttering until about 3 months ago, but I will definitely get the proper leaf springs for it as soon as possible. Now I understand what he meant to DIAGNOSE the problem with washers. I told the SHOP to try that and they wouldn't do it. They said that's not a fix. But, I'll have it back next week and I can do what ever it takes as far as diagnostics. Believe me, I can't afford to just slap parts on it. I'll need a crawler first and maybe I can do it. I still have the physical limitations to deal with. If the washer diagnosis works then I'll know what the problem is and what I have to do to fix it once and for all. This has all been a nightmare because basically, I have just been the messenger. Each message I pass onto the shop, gets shot down. I couldn't just go out to the garage and try the possible solutions mentioned myself. I'm only out $100. That's labor, spark plugs and a whole lot of grief.