Sounds like you have a bad U-joint in your drive line. My Ram had the same symptoms, rear end vibration. I have replaced both of my U-joints within the last 18 months

 

Thank you everyone for the advice so far. Is the method SeanW727 mentioned for testing the U-joint the best method or is there another one?

 

You can also go back and forth from forward to reverse.... IF you hear a click/clank noise... another sign its bad.

 

I tried to move the driveshaft by hand and couldn't get it to budge. I shifted from reverse to drive and have an obvious clunk. It's more prominent going into reverse rather than drive. I can't tell if it's coming from by the transmission or the differential (I believe these have 2 U joints). How hard of a task is it to replace the U joints? I have mechanical experience but mostly on FWD imports.

 

IF your driveshaft/ujoints arent rusted to hell its actually not to bad. I recommend replacing all of them at the same time, depending on if u have a 1pc or a 2pc driveshaft you either have 2 or 3. Btw, when you said it clunks more in reverse.... I am 99% positive its your u-joint.
If you have a benchvice.... or a c-clamp...u can usually get them out... Or access to a press. If not autozone/advance auto will rent you the tool for about 95$ and then u get your money back when you bring it back to them.
IF you google Dodge Ram U-joint replacment... when i did mine a while back there were a few articles on there... you should find some good info.

 

They don't look rusted bad to me. Obviously a little rust but nothing serious. Do you recommend not worrying about this right now? I would have to look at my driveshaft I have a feeling it's the 2 piece driveshaft because it's a long bed truck. I'm just trying to figure out how serious this is.

 

Lets put it this way its a 15$ part. That if it goes completely can cause your driveshaft to spin around and take out anything in its path underneath your truck...

 

Thats kinda what I expected. Thanks

 

bump? did it turn out to be the u joints tegguy? i'm having the same problem at 40mph even after a re-balance on all four wheels and a rotation on my 04 qc 4x4. I tried checking the u joints by rotating the drive shaft but the only play comes from the output shaft out of the tailshaft and the yoke mount going into the differential, not out of the u joints. I've had my truck for over a year now and it wasn't present when i got it but it has gotten progressively more apparent, either bc i notice it or bc it's getting worse..