Check the U joints , and drive shaft , for damage and " phase " between the front and rear haves of the shaft .
Some times if the rear section of the two piece shaft is removed for towing of the coach , and it is not properly re-installed causing a vibration .
The change of sound on bumps maybe due to bad drive shaft angle , that requires the center steady bearing, mount be shimmed to correct.
NOTE: 30 MPH start of the issue almost completely rules out a tire problem ; but you could have a look for something wedged between the duals .
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99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.
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