2000 GMC 1500 transmission question

[68c10owner]

Looked at a truck a friend is selling. It was his cousin`s who passed away. I drive it today and 1st gear takes forever to shift into 2nd. Then on the freeway at 65mph I stepped on it and it dropped 2 gears, revved high and went nowhere. The fluid is full, red and not burnt. I think its got a issue but my buddy thinks that maybe since it has a hypercharger his coulsin messed with the shift points. I can get the truck pretty cheap and even if I put a tranny in it its still a good deal but curious what you guys think.