I will. Going to take some pictures of the driveshaft and post in a few hours.

If it ends up being in the rear gears from the install, I might just have to deal with it for a while. The axle bearings and all components were reused from a rebuild and add of a limited slip about 6000 miles/2 years ago..

Maybe the gears/carrier & bearings just didn't take a good set this time,could be somethings off there.

I agree in theory but every driveshaft that I can recall had some slight play at the trans. The splines are not an interference fit, they do slide as they rotate.
My Charger has the best collection of parts of the cars that I own and even it has a small degree of slop. Zero drone, zero vibrations other than the engine.

Here are a few photos. After checking front snout slop again, there really isn't any movement. I'll try and post a video.

Do you have a angle finder, compass?
If so measure the pinion angle and driveshaft angle and let us know what you find yours are, if you don't have one or can't borrow one go get one (beg, borrow or steal one ) and check your pinion and driveshaft angles with the car on the suspension like it driven
In the sideways pictures of your driveshaft and pinion U joint is the rear end on jack stands or is the rear end hanging down on the cars rear suspensions?
Me thinks your driveshaft is to short The yoke needs to be in more when the car is on the suspension, so if it is not on the suspension in your pictures ignore this comment

No sir, not on the rear tires. On jack stands with the rear suspension relaxed and jack lowered.

Here are pics of front and rear yokes, perfectly level and camera level, with rear on ground. Rear yoke might be at ever so slight steeper angle than front, but is very miniscule. I'll try to properly get the reading though...

Edit: pic of rear yoke is off a bit. Really at same angle as front.

the first thing I would do right now is pull the rear ujoint caps & move the shaft all the way forward & measure the distance back to where the caps seat in the yoke.

I wonder of I am seeing it right. It appears that the yokes are not in alignment. They have to be within a few degrees of each other to avoid vibration. The u-joint connection speeds up and slows down at every rotation. I'd expect trouble if the u-joints didn't both do this at the same time every rotation.

That picture of the rear didn't come out right; the camera is tilted a bit. If angled the right way, the tires are level with the ground.

I will check that out. About 1.75" exposed of snout and no less than 5/8 of snout is engaged in trans.

If the car is on the ground in the last set of pictures the driveshaft is about an 1" short.

The car is on the ground. Yes, the driveshaft could be a tad short, but I am not grasping why it's now an issue when it wasn't prior to the gear swap.

nothing was changed that would move the housing a tad rearward?

Not since the gear change. Motor mounts done a month before the trans swap, which was a year ago.

Can you verify both U joints are in line on the driveshaft? The driveshaft looks to me to be too short...If it's not into the tail shaft bushing far enough it does wobble and will cause a vibration. The splines and the tail shaft do not control the yoke as much as the bushing in the tail shaft housing does.

Yes, both u joints are in line with each other.

I was told you want the driveshaft to be able to slide 1" forward before it bottoms out in the trans.

There is a tool available to R&R the rear trans bushing without pulling the trans. It cost about $150. I bought one and did mine chasing a similar high speed vibration. I have yet to find it, and it only occurs above 95mph.

Where the rear pinion bearings changed? Maybe that front pinion bearing is bad, nut tight? That might be the place to check since the "noise" came after the gear change. I thought I read you replaced it all at the same time.
Also since you had to change front slip yoke, is it bent? Have to watch it turn running in gear. Only way to check the driveshaft for harmonics is to try another one.

No, nothing was changed when the gears were changed to 3.91 recently. Everything was replaced in the rear a year prior when the sure grip unit was installed with the 3.21 gears previously, so nothing was touched this past time around.

The front slip yoke was sourced and put on the front of the driveshaft back when the 4 speed was put in early last year. Nothing appears bent and it all functioned without issue with the 4 speed/3.21 combom before going to the 3.91 gears.

My dad is checking for engagement of the driveshaft into the trans today while I am at work, making sure that the driveshaft isn’t backed out more than .750-1.000” after pushing it in as far as it will go. Will report those results later.