What's happening is that at speed, the driveshaft is turning fast enough to cause the vibration - this is most likely why it only occurs in 3rd and 4th. When you are on the gas, the torque being transferred to the rear axle keeps the pinion at the right angle, or at least close enough, thus giving you no vibration. When you remove your foot from the gas, the engine is braking the Jeep through the back tires. This causes the pinion to point down slightly more then it would normally. This is increasing the angle just enough to give you the vibration that you are noticing.
You could put a 2-3 degree shim between the leaf spring and the axle to point the pinion up a little more, but I would suggest you install a Slip Yoke Eliminator, especially if you 4 Wheel at all.
E-mail me if you have any questions.