Help

[Chevylife4ever]

Hi guys I hope that all of you can help me out 2003 gmc four wheel drive 5.3 liter  wouldn't fall out of a tree if i  even hooked too with a race truck changed plugs  fuel filter & pump took the cats out too still won't hardly pull a small hills at all just started acting this way any ideas on what it could be 

[txab]

Did you check for stored codes?

 

[richard wysong]

timing chain?