What's Causing the Knock?
 

I have a 1998 4x4 ram 1500, 5.2 engine, 5' lift w/315/75/16.
It started with a little knocking and when I would accelerate the knocking got faster. I removed the to look at the u joints. Front looked good, the rear one looked good but a little stiff in movement. Took it for a ride in 4H without the shaft, knocking was gone. Checked the fluid in the rear and the level was good. A friend observed the drive shaft and rear when I would shift from drive to reverse and back. He said that the drive would move upward 1/4" and that my problem is the rear. What else can I do to find the real problem here? If it's the u joint I can fix, if it's the rear what do I need to fix or just go to the?

(When truck is in neutral, there is about 1/16" play moving the drive shaft upward. It's as much as the front drive shaft).
Looking forward to your feedback.

Sounds like a wheel universal joint or the ones at the transfer case on the front driveshaft.

I was thinking the same thing...u-joint going bad

Well usually i don't think u joint when folk suggest knocking but it could be. Usually you can inspect them and see they are bad most of the time if they are knocking that much. Are you sure its not comming from your engine? I have a 1999 and the engine knocks when it gets warm. My temp gauge says it good but i wunder if my thermostat is sticking closed some. It does not do it all the time just now and then and especially when warm out with plow on. Drive line U joints usually start out clunking a little when put in gear then progressively get worse. Front axel joints make racket when turning and get worse.. Hope its a ujoint and not somthing else..

U joints are good on front and back drive shaft. I lifted the rear tires off the ground and noticed that you can push the tire inward making that knocking noise. So I believe that the wheel bearings are bad. Does anyone else agree?

Wheel bearing or ball joints.

Rear axles will move in/out 1/8th to 1/4 inch. That's normal. If it moves up and down, that is NOT normal.