Ok I have been trying to track down and fix a cyclic vibration on my truck for the past year and it's been driving me crazy. It happens at all speeds over 25-30 and gets worse the faster you go the trans does not slip and ***** fine. Also there is audible clicking coming from the engine bay when under load but i'm not sure if that is related. I have searched many "vibration threads" so I have changed all the usual culprits. So far here is what I have done so far.
-All front end suspension components
-Different Rims/Tires
-New injectors
-New Spark plugs/wires
-New Ignition coil
-New Fuel pump/ pressure regulator
-New one piece drive shaft
-New upper/lower intake manifold gaskets
-New Front/rear brakes (rotors and drums as well)
-Both rear axle shafts have been swapped
-New torque converter/flywheel
-New DPFE sensor/hoses
-New Camshaft sychronizer (old one broke)
-New trans mount and cross member bushings

I have taken it to a highly rated shop twice for diagnosis and both times they could not track down the problem. I also took it to a aamco transmission place twice and they also could not pinpoint the problem either and both directed me at parts I have already changed.

So my question is does this seem like a transmission problem? I have the 4r44e btw. Any advice would be appreciated.

 

I had a very similar problem, it was my drive shaft, it could not be balanced and had to get a new one,what year and model 4x or 2 wheel

 

its a 96 3.0 extended cab with 2wd. It originally had a two piece drive shaft that I swapped for one piece drive shaft from a pull part junkyard, but I suspected it might be bad so I had one made a few months later. So I have a new drive shaft with new u joints but it does have the original slip yoke i got from the junkyard but it doesn't feel like there is any play if I try to move anything on the drive line.

 

Can you jack up both rear wheels, jacks/stands under the axle so suspension stays in the same place, and then run truck up to 30MPH on speedo.

That would tell you if it is drive train or front wheels.

If drive train, pull drive shaft, put trans in drive and run it up to 30MPH again, if no vibration then issue is in the axle/rear wheels, if you have a vibration then engine/trans issue.

"highly rated shop" should have done all this

 

I had been worried in the past to run it while it was jacked in fear it might vibrate off up but I have a motorcycle jack now and i'll use that and some large wheel blocks and test it myself this week. I assume the shops did run it on the lift but i'm not sure they had the suspension loaded. I usually have a topper on but I have it off right now and it seems to vibrate worse the less weight is in the bed. I know the leaf spring bushings are somewhat cracked but I don't think it would cause this kind of issue.

Also is it ok to run the engine without the drive shaft in wont this leave the output shaft free to wiggle around?

 

You need to be IN the truck when back end is off the ground and powered, main reason is safety, but secondary is to feel the vibration

If output shaft wiggles then you found your problem :)
drive shaft sleeve is just to pass power, it is held in place by the output shaft not the other way around

The leaf springs align the rear wheels with the front wheels, I assume with new rims and tires the shop did a 4 wheel alignment, it is pretty standard now.
While it isn't as exact just measure from back tread of front tire to front tread of back tire on each side.
Use tape measure and put a mark at the same height on all 4 tires, then measure.
Obviously front tires need to be straight forward, and make the mark on tires low enough so body isn't in the way of front to back measurement

 

there is one possibility i noticed you did not post. it is possible for brake rotors to wear unbalanced. not common but happened to me when i had a buick century and a mercury topaz.

 

Yes i was considering this the other day. I have changed the rotors and the pads but i haven't gotten new calipers so after i test the truck with the rear tires lifted and run it i should be able to tell if its in the front end or not.

 

Loose plastic body piece or air deflector?

 

When I had this issue, I put floor jack under rear end and at 45 mph you could see drive shaft vibration, also could feel it in drivers seat