Your shocks are worn out. Replace them with Rancho RS5000 series. Vibration will be gone.
I say this because I chased a similar problem on the Dakota. When I finally replaced the shocks, problem gone. Smooth as silk. Magic carpet ride ever since.

 

I forgot to mention that I had replaced the shocks but the problem persisted. Funny thing is, it doesn’t always occur.

 

What shocks were put in, what are the replacements (make/model)?

Potential sources of vibration, you have covered off in your first post. Tire balance, tire ply defects, driveshaft dent or thrown weight, u joints, transmission mounts, sticky brake calipers, sticky ebrake components, .....

Besides those, you would be looking for components that are to inhibit resonance in the suspension and/or frame. Shocks and bushings are the major components to stop that.

 

 

The Reflex. Had tried those on two of my previous trucks, a GM 1500, the Tahoe. Took them off within a week on each of them. I found the reflex to be too stiff around the middle. In normal driving conditions they cause alot of ride chatter in the rear. Absolutely hated them.
A very strong recommendation is to try different shocks.

 

 

Rancho 5000 series. Over the years I have put on my trucks the Monroe (Sensatrac, reflex, HD), the KYB, the Gabriel, ... all of them I was never really satisfied. Once I put the Rancho RS5000, I have never looked back. Every truck I have had since, the first thing that gets changed and installed are the Rancho RS5xxxx, I just do not look at anything else anymore. I do put Biltsteins on the SUVs and the cars, the Rancho on the truck. Monroe goes into a Prius. Rancho goes into a truck. Reasonable price point (price does not equate quality btw). Great ride. Great shock all round purpose.

Sensatrac - too soft, felt bouncy and out of control most of the time. Ride with no shocks at all and it will probably feel better than the sensatrac. NOT a truck shock.
Reflex - No actual "reflex" release. Sharp chatter over short bumps such as tar lines or washboards which then resonates the whole truck. Might as well have a straight solid bolted axle. Kinda ok on long slope dips, loopy humpy roads, and such. Hated them.
HD (Gas Magnum) - Actually fairly nice. A bit too soft when box is loaded though, results in a blown shock. A good shock but not as HD Magnum name implies
KYB - like the Reflex, much too stiff over the whole travel, no matter the road condition. Like putting solid rubber blocks in.
Bilsteins - awesome all round, softer than the Rancho. Much more pricey though
Rancho - awesome all round "truck" shock, all road conditions. Plants the truck. Always feels in full control. Excellently priced right on point. Magic carpet ride and happy wallet pretty much guaranteed.

Depending on where you bought and when you bought the Reflex, there is usually a money back satisfaction guarantee. Hopefully this is a situation you can leverage, so you can just pull them, return them, swap for the Ranchos and actually get some cash back.

 

 

PS: I overlooked making a bullet on my experience with the Gabriels. These are also very good option. They ride much like the Rancho only I noticed them to be slightly firmer, a bit less cushy round the middle. The Gabriel are not as commonly known or to find. I suspect that is simply because of less marketing. I liked them. Just that they tend to be a bit more expensive and harder to find in stock, meaning having to order them in, depends alot on the region.

Hope any or all of that helps! Let us know what you decided to do and the results.