My jimny is really struggling to get up hills and feels like it’s loosing power

Hi guys
I’m new to the world of jimnys I just picked up a 2001 jlx a few days ago and it’s a really fun little car but it seems to be struggling up hill and loosing power if I go higher than 4th gear?

I’ve checked the obvious stuff like spark plugs air filter etc but it hasn’t made any difference I can’t figure it out?

There does seem to be a kind of swirling noise coming from under the car when the revs climb and gets louder and louder the more you push it? It almost sounds like air escaping from somewhere?
If anyone can help me figure this out I’d really appreciate it
Cheers

Yes it’s really struggling to rev over 4000 under load

Or the rear silencer may have collapsed internally

Update I pulled the cat off and there seems like there’s barely any flow through it so going to change it in the next few days so hopefully that will sort it

Has putting a new cat in made any difference?
Where did you get the new cat from?