I can picture the sleeve you describe. The IT40 I work on has both speed & direction thru the shifter tower- both actuated by a single stick (3 forward, 1 reverse).

Is this a new customer? Is this a new unit for your customer? Has the usage changed for this unit? What oil is in the trans & where do you have the oil level set at?

Can you raise the drive wheels off the floor & run unit in gear with the shift tower removed to check flow pattern of oil?

If the unit is designed with this sleeve- there must be more at play for it to fail.