When only off the gas and deaccelarating noise and vibration.

Hey Rhonda,
No, if it was snug against the cab it would rub, make noise, cause a rust point, etc..etc.. the bed must be sitting on the frame by itself..it allows flex and movement when it's loaded and when the truck's frame is tweaked while off roading.,, this couldn't happen if the bed was against the cab.
Hope this helps...

Bad Wheel Bearing = Vibration while braking? - DodgeTalk : Dodge ...

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2006 dodge dakota p/u - Discussion on Topix

1996 Dodge Ram 3500 diesel Standard Payload, lbs.ext. cab long bed 5151, ext. cab short bed 3915, reg. cab long bed 5340, reg. cab short bed 2372.---------------------------------------------------------------------------------------
Multi-Di@g Access J2534

It is most likley your U-joints. check to see if they are tight and lubed. Newer U-joints are sealed, and not serviceable so they need replacment. It could also be the driving force of the driveline pulling your vehicle's suspension down, causing a bushing to sqeek. Also check the carrier berring if you have a crew cab long bed.

Do a complete tune up, plugs, wires all fluids & filters. Consider using synthetic oil and gear oil, and keep your tire pressures up to specs.

This is possibly a tire out of balance.

take it to a service shop you trust,or take it back to where you bought it and get the service records on the truck,and get the phone number to the past owners, and also get a carfax on the truck,I hope this helps you.

according to the owners amnual the max towing weight for a 1999 ram 1500 quad cab wit automatic transmission is 7600 lb.
with either 5.2 or 5.9 v8. for a reg cab it is 8000 lb.

yah the 12/16 was a lie dodge never got that with there gas 4weel drives the average is right close to 10mpg i have owned dode all my life. and have never gotten over 13mpg with them. I sold my 97 2wheel drive 5.9 gas and thats the best i did withit was 13mpg and that was with the 285/75s on it you have to remember that the tires that big actually travel more distance than what your odometer is going to read. But right around 10 is average for the 4wheel drives

No, the frame is different.