Becareful who you let build your nv5600 sence mine failed I've manage to get it pulled out, had metal in the oil, pulled the tail housing and found that unit was built with worn out parts, the main shaft nut was put on with a chisel and almost broked and not tight, this unit came from cummins performance parts i got took dont let this happen to you

15 minutes ago, Mopar1973Man said:

I've towed may times over 10,000 pounds of cargo (excluding the trailer weight). Fact is the two upper gears you use the most are on a 6 speed are the exact same gear on a 5 speed. Nothing gained and 5 speeds are cheaper to rebuild and no 5th gear nut is not a problem as long as the nut is properly torqued. IIRC the torque for the 5th gear nut is like 600 ft/lbs. So all these redneck youtube video beat the nut on with a hammer and chisel are going to fail anyways. Yes it's going to cost in tools to be able to do it properly but in the end the transmission is very strong. 

 

(Just 1-ton hay bails there are five hay bails on the trailer)

My thoughts too on the 4500 I've got the tools and skill to build a 4500 I've seen many 4500s take some serious abuse and hold up, I've not had good luck with the 5600 so instead of buying another one and can swap to a 4500 for the same or cheaper and give that a try, I know towing don't kill them if driven correctly