why is my truck have loss of power? on 1996 GMC 1500 Pickup

when you give it gas it act like it wants to go and sounds like it has power but wont go over 10mph, I though it was the tranny going out but after putting a new one in it still did the same thing, what could be casing this?

Asked by for the 1996 GMC 1500 Pickup
Automatic or manual? 2 wheel drive or 4WD?