JD 450 bulldozer won't move in low

[Bull3333Dog]

My JD 450 bulldozer will not move in low. Reverse and high work fine but the bulldozer will not move at all in low. If I start in neutral and put it into low, nothing happens. If I move it to reverse and then switch to low, the tracks try to move and then stop.
I tested the hydraulic pump and I have 180psi. When I shift I have 60 psi like the service manual says I should have.
What other tests could I do to figure out why my bulldozer will not work in low - without actually having to open the transmission cover to see if the clutch pack is working or not?