Today was raining and air was moist. It smoke like crazy. Got glow plugs last year from BD customs. Maybe they went bad.
You need to check resistance on every plug, then also check to make sure every harness connector is showing 24V. It's not unknown for wires to look good on the outside but have high resistance or damage internally.
The fact that it goes away when you increase the throttle or after it warms up suggests it is a cylinder temperature related issue, which is what the glow plugs do.
I suppose there might also be a cylinder with poor compression when cold, or just lower than the others that could cause the fuel to remain partially unburned until warmed up.
My truck smokes badly when cold, because I still haven't gotten around to changing the glow plugs on the passenger side. Lots of smoke, then tapers off when warms up. Also decreases if I press the accelerator pedal.
Glow plugs are one of those hit or miss items. They should last for a decently long life, but all it takes is a couple times where you turn the start switch, turn it off, and back on again too quickly to start burning them up.
Just to be clear, you said this truck has 3k miles on it since rebuild, and the previous owner put 2800 of those miles on it?
Add to that, the start box and glow plugs were replaced 8 months ago, was that by you or the previous owner?
Has it smoked like this the entire time or did it just start recently?