95 maxitorque 13 speed question.

I recently bought a 95 mack dump truck with a Mack 13 speed it grinds loudly when splitting down especially in high gear if you shift to neutral before you shift then back into gear no noise. I have taken the back box off my Eaton 18 speed and put a synchro disc in high low but the splitter didn’t have any. Does the Mack transmission have one on the splitter? Or what do you think is wrong with it or do i just not know how to shift a Mack iv never had one before. Thanks