Well, it will probably be 1 of 2 things. It could be as simple as the linkage is not allowing it to get all the way into gear. The car will still move, but the gears have not mated all the way together and it is not getting past the blocking ring inside the transmission between the synchro and the gear. It would then pop out under load. The little u-joint between the shifter and the transmission might be worn out just enough to cause this. It will feel as if the shifter knob is pushed all the way foward, but actually the rail going into the back of the transmission still need to be pushed in just a bit more.
If the linkage is getting it into gear ALL the way, it is going to be ugly. The synchro, gear, blocker ring, and the slider will probably have to be replaced. The parts $ are not nearly as bad as the labor will be.
Usually it is a linkage issue, unless you have jammed it into reverse while the car was still moving foward several times.