[Tom Wesenberg]

If everything is right, it shouldn't take too much detent pressure to hold a straight cut gear in place.

First the flywheel cover must be shimmed to within .006" at 9, 12, and 3 o'clock positions.
The pilot and front tranny bearings must be snug so the shaft can't have any sideways movement.
The center bearing and rear tranny case bearings must be snug to keep the output shaft from excess side movement.
The gears on the shaft must be a snug fit so they can't move side to side.
The cluster gear bearings and shaft must be good to prevent side slop.
None of the gear teeth should show wear in a V pattern. In other words the edges along the length of each tooth should be perfectly parallel, even though the leading edge has a bevel to help with a smooth engagement.

I have a 1949 Chevy 1 1/2 ton truck that will walk out of granny gear after about 30 feet, but I'm never in that gear more than 10 feet, so I never investigated the cause.