Manual transmission question
 

When I am backing up the 70 wants to pop out of gear. She will go maybe 20 feet and pop out of gear every time you back up.

What causes this?

This was happening to me too. I was told from some guys on here that it could be a bent shifter fork or worn gears (from jamming into gear).
If you force the shifter, this can bend the shifter fork. When I serviced my rearend I found the shifter fork bolt was a little loose. I tightened it up, and haven't had any trouble since. Good luck!

My 100 jumped out of gear in reverse. The bolt that held the shifter fork to the rail was loose. adjusted it and tightened it down and worked fine. Fairly easy to do if you split the tractor.

you see the 1st R fork break allot. it usually cracks to the bottom side. if the bolt is loose or the fork is very loose on the shift rail check it for being cracked