The truck started this new thing while in O/D with the converter locked a few months ago... At about 45-55 mph and trying to accelerate it seems like it will slip for a second then catch jarring the truck. It will only do it thought at a low rpm. You get over about 50-55 mph you can accelerate as hard as you want and it will not do it. It seems like it is trying to downshift, but the converter wont unlock. It will finally downshift when I am at about 3/4 throttle. If I tap the brake it will unlock the converter though. I don't know if it is the same shudder alot of others are having an issue with or if this something different. A shudder seems like a vibration to me but this jars the whole truck. It has gotten worse the past week or so.

I had the fluid exchanged yesterday with Mobil 1 and it helped a little but not much. Does this mean a new tranny is in order? Or perhaps something simpler?