Transmission "Lag" between 1st and 2nd

[BigZed]

Hey guys,

 

Not mechanically inclined to bare with me. I'll try and explain what is happening as best I can.

 

From a stopped position I accelerate between 2000 and 3000rpm. First gear is fine, when the truck decides to switch to 2nd gear there is a lull for a second. Sometimes it feels like the engine revs a little higher out of nowhere, then 2nd is selected and then I get a smooth acceleration through all the gears. I mostly notice this when Im entering a highway from the ramp and I'm beginning to accelerate.

 

It kind of seems like the truck is trying to find 2nd before it actually selects it. If that makes any sense. During this "pause phase" I definitely lose forward momentum.

 

2010 Sierra All terrain with the 6 speed. Swapped the transmission fluid out at 100,000km. I was experiencing this behavior before the fluid swap. Truck has 105,300km on it now.

 

Is this a transmission issue or maybe a fuel filter/pump issue?