Very low speed shudder in drive but not reverse

I have a 99 V6 4x4 TRD Automatic and at very low speed ie in a parking lot or when just starting out slowly I get a shudder. It doesn't happen if I accelerate more quickly. I can sit still with my foot on the brake and press the gas and get the same thing. If I shift into reverse hold the brake and press the gas it is smooth as silk. I have replaced the tranny filter and flushed all the fluid with Dexron III. It shifts very well through all the gears both up and down. It is just the first couple of seconds from stand still.

Any ideas?

 

Just as a reminder, The problem can be produced when the truck is not moving. However, I have replaced all engine and tranny mounts, replaced the center support bearing on the drive shaft. I have replaced the plugs but not the wires or coils. The fact that it is smooth in reverse seems to point away from an ignition problem though. I don't know though, does the computer do something different with the firing in reverse? If so, why?