Engine/Transmission (1998.5 - 2002) NV4500 trouble

[ChuckV]

I have a 2001 extended cab, long bed, 2500 with a NV4500 and a Jacobs exhaust brake. I tow a 33 foot 5th wheel. When I go down hill in 3rd gear the transmission pops out of gear. Does anyone have any ideas ? Thanks, ChuckV

 

[GarrettPetersen]

A posible Syncro maybe