955H PTO slips out of gear?

Hi, I have a 955H Track Loader, S/N 60A10479. It has petal steering and does everything I need it to do, runs good, no smoke, undercarriage is about 50%. Not long ago I hooked up the backhoe that came with the machine, it was like new except for hoses.
I replaced the hoses and the hydraulic pump and the back-hoe works fine as long as it faces downhill? It seems the PTO stops when you are on the level or not facing downhill?
The PTO works when you put the Hi/Low range shifter in neutral and the transmission in gear (reverse). Any ideas?
Gary

Need to post this in the discussion section.
From what you describe seems like you have lost the snap ring that retains the pto shaft drive gear in the range transmission. Allows the gear to walk out of mesh in anything but down hill position.

Would also be a problem in low range when the double gear is not holding the pto shaft drive gear in position.