vibration on decelleration after hilux conversion

hello. I have just put a hilux diff from a rn25 in my mk1 and the you can feel the gearbox rattling on decelleration only when you are at around the 60kph mark. Now I have machined down lowering blocks on an angle to make sure the gearbox and diff are within 2 degrees, I have had 2 custom tailshafts built up as the first 1 was sticking out too far from back of box, 2 different companies and they were both balanced. I have also replaced the gearbox and it has the same problem. Has anyone come across this before as I have seen what vibrations in a gearbox can do and its not pretty.