Okay I've got, well actually my dad has a 1998 dodge cummins 2wd auto. All stock besides some fuel turned up. When we drive it, if it is on a strain or turning or in you floor it, it fells like something is popping up and rattling on the floor. My old first Gen did it but mine was just mounts. I looked under and got a prybar but nothing loose. Idk if it is the trans slipping or what. But the floor aggressively shakes when you gun it. But its like a small constant rattling when its warmed up. Any ideas???

weerd..

would need to get this on a rack with the wheels off the ground and see it in action

this might be a longshot but check your front drive shaft cardan joint and u joints

actually if any drive shaft has a problem the noise and or vibration can transfer pretty good to the cab floor

verify your front drive shaft is not locked in 2wd and check the joints

please update , this sounds like a interesting diagnosis

good luck
scott

So I should put a jack under the rear end and try it? And there is no grant drive shaft? Its a 2wd.

i would check u-joints and the carrier bearing if it is an extend id cab.