I have a 2004 TJ 5 speed manual with the NV3500 gear box. I bought it in March, 2019 and have only put 3,400 miles on it but driven it enough to get a feel for it's unique characteristics. Changed all the fluids when I first bought it and used Amsoil Synchromesh fluid for the trans.. Drove around for approx. 2,000 miles and always had a grind if I shifted too fast from 1st to 2nd. Switched to Redline MTL, same issue. I have just learned to live with it by giving about a second or two pause before shifting from 1st to 2nd. From reading other posts this is not out of the norm for that trans. and it is something I can live with. There is no need to replace this trans. at this time as it works fine. If the time ever comes I would look at an AX15 or The Terminator from Rockland Std. Gear. Interested to hear from others with the NV3500 trans. to compare notes.