I have an 03 4dr long bed dually with the 48RE A/T--when towing a 37ft 5th wheel camper and i allow the trans to go into o/d below 50 mph it makes a grunting sound==if i remember to lock o/d out until 50+ no problem. The truck or trans have no mods ---73000 mi The o/d unit was removed with trans still in truck, everything looked brand new Any ideas whats causing the grunting sound????

 

I supsect you are stessing the converter clutch and it's slipping/jerking as it goes into OD Lock up. It may very well be time for a fluid change and band adjustments. The 6 cyl is lugging at under 50 MPH in OD. With that kind of load, I recomend delaying OD till 60 MPH.