This truck has an odd shake in the rear of the vehicle in 4th gear at speeds between 30 and 40 miles per hour, which feels like it has two bent axles or a slipping clutch. Brand new leaf springs, tires, brakes, clutch, alignment, and exaust were recently installed but the problem didn't change a bit. After several different shops performed close inspections of the entire driveline, rear end, and the wheels, no one including Nissan can figure out what is causing this problem. The shocks are Gas Filled Super Sports for Off Roading and deliver a very stiff ride at low speeds. The rear end makes a slight whistling noise when the gas pedal is at a normal cruising speed. The driveshaft and u joints are very tight, although the tranny mount seems slightly loose. Could the shocks be creating this effect? Nothing else aside from a slightly loose tranny mount seems to be possible. To compensate and smooth out the ride, the engine must be accelerated above 3000 RPM or cruising above 40 miles per hour, otherwise it feels like the gear ratio is too high for RPMs or speeds below that range. Update: after driving the truck cold this morning the problem also happens in third gear as well as 5th, when crusing between 30 and 40 miles per hour up to 3000 rpm. Could it be due to a high rear gear ratio or the Gas Shocks?