On vehicles with a manual transmission, worn shift bushings can cause the transmission to pop out of gear, especially when coasting down in 1st or 2nd gear. Replacing all the shift linkage bushings is pretty straight forward and not expensive and will commonly correct this concern.
Car Problem Reports
Toyota Pickup Worn Shift Bushings Will Cause Manual Transmission to Pop Out of Gear
Toyota Pickup Problem
Drive Train Affected: 4WD
Visitor, 1992 Toyota Pickup, 4WD, 260,000 mi
pops out of gear when on uneven terrain
fixed by staying on even terrain ..... or holding gearshift in place