Dodge Ram 1500 lack of power

I have a 1999 dodge ram 1500 v8 5.9 magnum with a lack
of power. It always feels as if it's going uphill. It's an
automatic and when I start out itll start shifting weird as if
it cant decide on a gear, not exactly slipping, and then will
choose a higher gear and just be bogged down whole time.
Although when I put it in first itll have plenty of power and
when I shift to second itll shift fine and have power but
when I shift to "D" itll get bogged down again and have no
power.

My buddy who's decent with cars thinks it is the torque
converter but my mechanic who I called thinks I might just
have a clogged catalytic converter. I need to be sure which
it is because I've already put a ton into this truck and need
to fix it so I can sell it.

Any ideas? Could it be something else?

Thanks