I have a 2005 2500HD with an Allison

lately I have had a "snap" when leaving a dead stop or right before it comes to a complete stop. It is like the transmission is down-shifting into 1st gear. I tried it in 1st gear only, and it still did it. It feels like a HARD downshift. I changed the fluid and filters in the tranny, but no help.
Has anyone else experienced this?

This is usually caused by a sticky slip yoke on the center of the drive shaft. Remove the shaft and clean and relube the yoke and yoke shaft. The grease that GM used gets sticky and holds the yoke, until you start off.

Feels like someone bumped into the back of the truck. :D

that is exactly what it feels like. Will have to try that. Thanks

on the same subject why is that when i pull my yoke (haha) it is wet with oil dut it still sticks the splines are not dry or goopy