When I back up and push the clutch in rear wheels lock up.

Took top off transmission and forks were worn. replaced with top from junk yard still have the same problem. Gears look great in Tranmission. Put in new clutch, flywheel and trans top with used forks still has the same problem.

When it first happen I was able to drive home just using 1st and 3rd gear. Now I have all the gears but it grinds when I shift to 2nd and will lock up when I back up. Stumped in FL.

1991 F150 4.9 124,000 miles 2 wheel drive

 

I think, if it was me, the first thing I would do is to pull both rear brake drums to check that the shoe linings and shoe hardware are all intact.
If that is all good I would turn my attention to the differential.

 

Went to the Junk Yard In Orlando and bougth a used 5 Speed for $150. Works great.

Looking in the Old Transmission I still can not see a problem with the gears but somthing was not lining up right.

At least it fixed. I love the F150 easy to work on