Any GM guys who know a 700 R4 really good

They had problems with the reverse valves in some that caused shudder in reverse and reverse failure like you're experiencing because the reverse valve can get stuck. An upgraded version is in all the kits now as far as I know. The bad news is, as far as I can remember, the reverse valve is not accessible unless you pull the front pump, because that's where it is. I've been in a couple but I cannot remember if the back of the pump protrudes enough into the case to access the valve after you remove the valve body. Since the top half of the valve body IS the transmission case, I don't think there's any room to access it. That's all I can recall. I can tell you this. I've seen a couple of times where guys were able to "unstick" that reverse valve by gently applying some gas pedal and shifting from D to R. I mean gently too like on high idle. Increases pressure just enough to sometimes unstick the valve IF that's what it is.