I travel a lot with my 1996 p38. 3 Months ago I was on the interstate and the muffler under the cab blew out. I had a shop put pipe in it’s place for the time being. It progressively started running worst. I thought the cats were stopped up, so I slit the exhaust pipe just before the cat to relieve heat if they were stopped up it would have made a difference in the run-ability I assumed. Nothing happened. So I kept driving. It starts up fine. The idle is high on initial start up but evens out as it warms up. When I put it in gear it still doesnt miss a beat. When I push the accelerator to go forward it will do fine up until about 15mph and about 1500-2000 rpms it starts to cut out and shake the whole truck and backfire. If I let off the accelerator it will smooth out until it reaches those rpms again. It will eventually gain enough speed but be jerking and sputtering and backfiring. It will not accelerate above 55mph no matter what. If I try to give it gas past 55mph it suffocates itself it seems like. The motor will stop making noise completely until I let off the gas. Please help me solve this problem. I can answer any questions if the thread is a bit confusing

 

Did you check the other muffler? Could be clogged with debris from the first muffler. Sounds a lot like clogged exhaust symptoms.

 

Is there a check engine light? Did you weld the slits back up before the cats or are they still open?