But runs fine once warmed up.

The plugs are new. I can replace the wires and cap and rotor, but I don't think that's it.

Nor do I think it's vacuum.

I'm actually at a loss - is there something that would cause a truck to buck when cold in 2nd and 3rd (and a bit in 4th; can't tell about 1st b/c not in 1st long enough to tell...), but run fine when warm?

Throttle body sensor perhaps?

Thoughts?

Thanks!

 

cold start injector?

 

Have you checked the actual timing, my 86' 22RE was doing it as well in first and second at higher rpm. Sometimes the the engines just get out of it's torque range for the selected gear, how bad is it bucking, I think for a while there mine did it because of a brand new engine install and my fuel pump wasn't getting it adequate pressure, took it n it's maiden voyage to the wilderness and opened it up a bit, truck runs great now. NE ways, try your fuel pressure