Help with the diagnosis please!

Two weeks ago while driving down the interstate, the 4runner started vibrating under a load, "gas pressed" when in neutral or coasting there is no vibrating.

After some looking into, I have replaced all 4 U-joints. Being that I am v8 and 4wd all time, I thought this might do it. Unfortunately I am still vibrating under a load.
1. Possibly tires, the Ridge Graps have been on for 70k. They need some replacing. But since it only shakes under load and not all the time I am ruling this out for now. I have a set of at3w's in the garage. Just haven't put them on.
1. Caliper seized, None of them are hot after driving, and that wouldn't be under load.
1. Diff's and transfer case oil, all full and good.
1. Slip yokes and u joints. All new grease is each
1. My next thought is possibly an axle?? They are Advance auto axles, been in there for years.

Anyone have any other suggestions?? Thanks for the consideration!

Update: grabbed my driver axle again while on ground. I can physically move it slightly and it make a noise. So, something is up with that. New CV tomorrow!

if the axle ends up not being the issue it wouldn't hurt to go over and check your tie rods and alignment. That was the source of my shudder/shake under acceleration: bad alignment with the toe being way out and worn out tie rods allowing lateral movement.