MudPumper

Okay, when driving at 40, 50, 60, or 70 mph, I get a deep pulsating vibration, but at 45, 55, or 65 it's not there. I cant tell even the general direction its coming from, just under the truck. It's a '95 3/4 ton 4x4 with a turbo diesel, auto tranny and 149000 miles. Any sugestions would be great.

RCW

Check your driveshaft for loose or worn universal joints, sometimes the only way to tell is to look for rust traces around the trunion seals on each joint. Also inspect the rear driveshaft bearing, and if there is any slop in either the mounting or the bearing itself, that will cause a rumble.

If it is none of those items, check your exhaust system for any leaks. The exhaust will reverberate and create noise harmonics at certain speeds.