Old thread but since nobody responded to danny k's skepticism...
That actually was a bona fide field fix to manage the problem of pinion seal failure in those FG--ST-11 models trucks without having to replace the pinion seal.
I installed the "drain back" hose field modification on more than a dozen FG20ST-11 trucks in our company shop back in the mid 1990's.
Most of the pinion seal failures occurred fairly early in the life of the truck and that fact is likely what inspired Komatsu and the transmission OEM to develop this radical approach to resolution of the issue. My guess is that there were enough failures occurred during the factory warranty period that the drain back hose mod was the easy way out for both corporate entities.
In our shop we would install the drain back hose and then put the truck back into service on the dock with Auto Trans Fluid in the differential. The truck would be used for 1 day and then return to our shop for a complete drain and fill of the transmission and the differential. Then after that the truck was just operated normally.
The only thing we had to do differently to the modified trucks was to drain and fill the differential along with the transmission when we did the 3,000 hour PM service so that we would not be diluting the new fluid with old fluid from the differential case. We also had a decal printed with the special instructions for maintenance which we applied to the lift cylinder hose guard plate where it could be seen when checking or servicing the truck.
Regarding the durability of the differential components being run in ATF vs. heavy gear oil: We never had a single failure of any differential components while those trucks were in use.
Some of the trucks remained in service accumulating well over 30,000 hours before the company began selling them off as newer model trucks were bought.
When Komatsu introduced the successor FG--ST-12 model truck it had a different transmission than was in the -11 trucks and pinion seal failure was no longer a problem.
Maybe somebody will find this fix helpful if they encounter one of these old trucks needing "the cure".