The transmission is misaligned.

Align the transmission.

The flywheel pilot bearing is worn.

Replace the flywheel pilot bearing.

The countershaft bearing is worn.

Overhaul the transmission.

There is excessive end-play in the countershaft.

Overhaul the transmission.

There is excessive end-play in the idler gear.

Overhaul the transmission.

The reverse idler gear is worn.

Overhaul the transmission.

The countershaft is worn.

Overhaul the transmission.

The clutch linkage is adjusted incorrectly.

Overhaul the transmission.

The mainshaft pilot bearing is worn.

Overhaul the transmission.

The gear tooth contact surface is scuffed.

Overhaul the transmission.

There is a lack of transmission oil.

Fill the transmission to the proper oil level.

The transmission oil is the wrong grade.

Replace the transmission oil.