Also, I don't know if this is applicable, but I had my SLS removed and replaced with coils when the airbags began leaking. This was done by the stealership about 20K miles and 4 years ago.

 

The 1590 is "ONLY" for the 3 Amigos, nothing else.
If the ECU throws a 02 code, it is a real issue and rarely if ever caused by another component. Again at your mileage, it is a little early to replace all 4 which should be done after 100,000 miles, and realizing the rear 02's don't have as much effect on the engines performance, I would still replace it if it were mine.
I see you keep bring up ACE, have it or not, it does not effect the engine and how it performs and you don't seem to have any codes indicating a vacuum leak.
So my BIG question is what is mis-firing, what codes are you getting indicating a back fire, if you have a mis-fire, who's wires and plugs have you installed?
A mis-fire can also be caused by carbon build up, burned valves and a failing coil so where are the codes?

 

I have no codes other than the P1590 and one for the 'bank 2 sensor 2' o2 sensor. I can't remember it off the top of my head, but its something like P017_ _. Thats it. I used bosch +4 plugs and magnecore 8mm wires. I thought the 3 amigos were only thrown in cars that have ACE. Thanks for your help Mike, any other ideas? I'd rather not drop the money on the o2 sensors right now and hope it fixes it, when the truck has more pressing needs like a new front end.

 

Fixes "WHAT", that is what I am trying to figure out? You talk about a misfire but have no codes or mention of when it mis-fires and what cylinder is having this issue.
Do you actually feel a mis-fire, if so when, and if it is a misfire, you really do need to have a code for it.

 

I feel the misfire between 32-45 mph, when I very gently accelerate. I must emphasize the "very gently." Its like a slight stutter, and lasts until I hit about 45 mph (not sure of the rpm), then goes away. The idle also is starting to be a little rough, although the tach doesn't fluctuate, I can feel the misfire. Those are the only codes I get. And like I said, the SES light will actually shut off intermittenly, after a few ECU cycles, then come back on for a few more. I'm perplexed. How is the P1590 related to the three amigos, and are the three amigos only thrown in Disco's with ACE?

 

Send me your number and I'll call you back before I head out for the morning.

 

I have half of Saturday to run tests on the truck. If it were you, Mike, where would you start looking for problems? You mention the coil pack, burned valves, and carbon deposits. I have a compression tester and inline spark plug tester, but what might be more helpful in narrowing down the cylinder that is causing the problem? As far as carbon build-up, I did the seafoam cleaning before installing the plugs and wires. I forgot to mention I replaced the air filter with a Wix filter at the time I did the plugs and wires. Also, is it possible my senses are wrong, and there is something else that might feel like a misfire, but in fact is something else?

 

It sounds to me like your torque converter is what you are feeling, not a misfire. Every so often mine will seem to "stumble" a bit in the same speed/rpm range yours does. Could be totally unrelated to the codes you have. Just a thought.

 

How do I check my TC?

 

It is electronically controlled, but what your'e experiencing could be the result of low transmission fluid. After I replaced the fluid and filter the shuttering stopped for me. There should be a description of procedure in the "sticky" section, but you want to remove the fill plug with the motor running on level ground. I think you're looking for something like a 2mm constant bead of fluid running out when full. Then replace the plug.