I too have a JD120 that is 7 years old at 182 hrs and it too slips in reverse and growls within the HST. The first thing that I found through one JD mower dealer was that the non-servicable transmission can require servicing with possible causes being loss of trans fluid through a missing or knocked off axle cap,the internal filter may need cleaning,and if it is left too long with the grumbles and growls it then needs parts replacing or I could replace the whole transaxle for $800.00. I was then two weeks later at a major regional JD dealer who gave me access to their workshop manager who sat me down and patiently fault found my possible problems through discussion. the end result was since my brakes still worked the transaxle was ok and that the problem would be with my seven year old drive belt and worn out tensioner / idler pulleys. Having too much fun on the yanmar to pull the L120 apart but he showed me his collection of L series belts and pulleys he has relaced and it made sense to me.... maybe this helps you but it is only my experience and fossicking through the web I found lots of reported L series trans axle failures but I have had a good run out of this mower for the work that it has done, and I may need also to service the transwarp drive after doing the drive belt and tensioner, pulleys...regards