High/reverse clutch or clutch apply circuit is the issue. It's the only component applied in both 3rd and reverse. Did you install the cupped orifice plug in the high clutch apply passage in the case? Did the trans function before the valve body mods? Was it rebuilt? If so did you air check the clutch? The bad news unless its a valve body malfunction the trans has to come out. The good news is the high clutch is cheap and easy to repair once the trans is out. Let us know.
Doug