B3000 Bucking at higher rpms

[04viking]

I have a 2003 mazda B3000 with roughly 51000 miles. I notice on hills that it "bucks" when shifting at rpms at or above 4000 rpms (note, it does this shifting from all gears on hills). Say i am cruising at 55mph behind a slower car and want to pass on the interstate. If i try to accelerate past the car rather quickly, it feels like it drops down two gears and then i get this bucking feeling as if someone is ripping it in and out of gear (note, on interstate, i only notice it around 55mph when i try to get up to 65mph and pass someone).

Since noticing this, i have replaced the spark plugs, spark plug wires, coil pack, fuel filter, and had the fuel injectors cleaned.

Any suggestions as to what i should check next?

Thanks