Why is my truck is backfiring over 2k rpm?

What is your question?

I bought a '00 k3500 dually and now it won't go over 2k rpm (at all), backfires and losses power if I push the gas to hard. I had the knock, cam and crank sensors replaced and the crank relearn can't be done since the truck can't hit 4k rpm. It sat for a good 2 or 3 yrs and was put back on the road in 2016 (at that time the truck acted like the rev limiter was set to 3k rpm and would backfire over that). The timing, rotor cap/ button, plugs/wires have been fixed/changed also. The truck also did not have cadilatic converters either, someone told me back pressure could cause my issues also. Its been suggested that either its my distributor itself or a fuel pressure issue. This is so far above my pay grade and I'm absolutely lost, any help at all would be greatly appreciated and thank you.

What makes this problem better or worse?

Too much gas or exceeding around 2k rpm.

How long has your 2001 Chevrolet Silverado 3500 had this problem?

Since I bought the truck (approx. year) but has gotten worse over last month.