Bouncing at 40 with a load?

[Bamtech1]

I’ve been reading the other threads about vibration and wheel hop, this new issue seems different to me so I’m asking a new question. I also posted in the towing and RV section, but that seems more focused on later model trucks.

1996 2500 4x4 quad cab longbed NV4500 8800 GVWR, 4:10s.
Just set the empty truck camper on, it’s advertised weight is 1611# with propane and water tanks full. Not much of a load.
What I’m experiencing is a violent bounce at 40ish mph, say 17-1800rpm in 4th. Seems torque-dependent, as I can go easy or hard on the throttle and it doesn’t happen. It’s a fast cycling maybe 3x/second, and ince it starts all I can do is let off the throttle. Feels like the bucking you get lugging too high a gear but faster, or like your foot bouncing on the accelerator pedal on a rough stretch.

 

[GAmes]

I've experienced that, and for lack of a better term I call it governor bounce. You can cure it by backing the gov springs back a click, or firmly planting the side of your foot against the floorboard when accelerating with a load. You don't indicate if you have installed a GSK but I have had it happen with both the stock springs and a GSK.

 

[Bamtech1]

Thanks, I’ll try that. I do have a GSK installed, and I’ve tried bracing my foot a little. This has only come up in the last 25 miles (one round trip to work) so I’m still feeling it out. I have found that I can baby it or romp on it through 40 and avoid the shakes.
Right now I have the driveline out to have a new slip yoke welded in. We’ve about decided that it’s the rear axle flopping. I noticed today that there’s just enough weight on it to contact the rear tip of the upper overload spring but not the forward end. It visibly lifts the nose of the diff a bit.