Thumps in explorers that do not vary with the rotational speed of the
drivetrain are often the full time 4 wheel drive misbehavin'
So if you have the full time four wheel, it may be over engaging the front
axle. This causes windup in the drivetrain which will be released as a thump
when something gives. Often the clutches in the transfer case slip with a
thump. In auto four wheel, your font driveshaft can engage anywhere from
about 10% I think up to 100%
Your thumps will go away on a dirt road, as the front tires can slip on the
dirt. Perhaps the new tires are grabbing dry pavement better than the old
ones, and that is what made your thump worse - your old tires were slipping
off some of the tension.
Hope you are still under warranty.
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