I  have a 2003 1500 Quad-Cab. It just turned 33,000 miles. Just last week,
for the first time I noticed a humming or vibration when decelerating from
about 60 mph down to about 40 mph. That's the only time it does it, just
when coasting down a hill in that speed range. If I give it a little gas
going down a hill, it stops.
The first time it happened I thought I had a flat tire, that's how loud it
was.
I have discounted the engine, tranny and exhaust form being the problem
because I threw into neutral when it was doing it on another occassion and
the vibration was still there.
I am thinking driveshaft, or rear axle or maybe a wheel bearing. It is very
difficult to isolate where it is coming from.
I only have 3000 more miles on the warranty so I will be getting it over to
the dealer shortly.
The truck has run trouble free till now, and this problem sure isn't normal.
I will let you know how it turns out as soon as I bring ot to the dealer.

Anybody else having a simlar problem with their truck?

Chuck