A couple of days ago I was bush hogging the levy of our pond. I backed up the slope in reverse low range. When I reached the top I pressed the clutch in and applied the brakes and let the tractor roll slowly back down the slope, approx. 30 feet. After the tractor came to a stop I released the clutch and the tractor did not move. The hi/low shifter feels like it will go into high and neutral but not into low. Anyways, no matter where I move the hi/low lever the tractor will not move. This is a 1971 Ford 4000 diesel with a 8 speed transmission. Does anyone have any info as to what the problem might be? Thanks !!!