Warren,
I agree with all the guys above. There are things that are easiest to do/check first which suggest. In short when in high gear the 2nd and high gear is directly engages to the main drive input shaft. which is always spinning . When alignment issue from wear and tear/flywheel housing shims missing, etc, it can cause the alignment of that to torque slightly which can cause the gear to walk off engagement while under power.
Bill mentioned the shift tower. I would definitely look at that first and look for wear. The shift ball ends get worn as well. Spring and plungers get weak... That's the easy one. check for shims between the upper ears ( where the accelerator linkage bolts) and make sure there's thin shims between the block and the ears of the flywheel housing. The shims account for the gasket difference on the back of the black to flywheel housing when it's assembled. If not used it puts undo stress and pressure on the housing and they will crack and cause issues. It is hard to see but with good light and inspection you usually can see. You may need to unbolt the accelerator linkage to take Look from the engine/firewall on the drivers side and you may need an inspection mirror slightly from the inside (floorboard removed) on passenger.
if in then remove transmission to proceed with checking the input shaft and bearings as they have noted
Keep us posted. It is nice to know what the outcomes are.
Larry Shepard