HELP, my truck's choking

[67Fleet]

I think it's an exhaust issue, but am not certain. My truck recently has been running rough. It idles fine, but seems like it's falling on its face at top end. It sounds as if it has an exhaust blockage or something. I checked the other day, got up underneath it and put my hand up behind both tailpipes to chk pressure. The pass. side was definitely blowing a lot harder than the driver side. A few weeks ago I noticed one of my pipes at the manifold was loose, and a nut was missing, so I put a new nut on and tightened everything down. Now I'm noticing this. What gives? There's some kind of spring actuated lever on the stock manifold, I checked that and it moves freely. Could it have broke and maybe be in the partially closed position? My low end is great, top end sucks.