1981 Toyota Pickup • 250,000 miles

I just bought this 1981 toyota 2 wheel drive for my 12 year old daughter. She and I are re building it together for her first vehicle. It has the 22r motor with manual shift 5 speed. The problem is that it will not stay in overdrive if you take pressure off the gear shift. 1-4 work fine but it pops out of 5th gear. What couold cause this and how do I fix it? My daughter and I apreceate all the help we cqan get. This project has been alot of fun for both of us but I dont know enough about transmissions to figure this out.
December 26, 2012.

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