Bill this does not sound like a caster problem to me. 2.5 degrees should be ok. 4 is a bit on the high side but only by about 1 degree...I like to use an even 3 degrees. I've learned you can move the wedges back/forward until you get the degree reading you want, mark them, drill new holes and go from there. Even if the new hole becomes more of an oval, I haven't had trouble with those shifting once things are tightened.
If it is belt separation you'll feel it no matter where the tire is. On the front, it will usually be felt in the steering wheel and you may see the hood shaking a bit, too. On the back it will usually feel like somebody is kicking you under the seat. That's usually a good way to tell if it is separation in just one or two tires. Good luck. I'll be curious...
