I’m assuming that by differential you mean that one aileron is trimmed up and the other is down to maintain level flight. A couple things I’d check are as follows:
-check the lateral balance (hang the plane by the spinner and tail wheel and make sure it hangs level. Add wingtip weight as necessary)
-make sure your elevator halves are aligned.
-try to borrow an incidence meter to make sure the wing isn’t warped. If it is seriously warped, you might be able to bug great planes into sending you a new wing

Tom