CAT 130G 9Y Series Transmission Won't Shift

Hello all, I've got a 1978 CAT 130G 9Y series grader with the 3304 engine that will not shift into either forward or reverse at any speed. Main Hydraulics work fine. So far I have checked:
Transmission Oil - Full
Transmission Filter - Replaced
Transmission pump pressure - low 250 PSI, added another spacer to relief valve group, now 300 psi
Shift cable - connected at both ends
Manual Modulation Cable - connected at both ends
Forward/Reverse Cable - connected at both ends

Any ideas you have...

CAT 130G 9Y Series Transmission Won't Shift

CAT 130G 9Y Series Transmission Won't Shift