Random Backfire

My 1991 Fuel Injected Ran 50 will randomly miss at idle. you can feel when the motor misses... sometimes one cylinder... sometimes a few..

Any ideas what causes that?

Sounds to me like you might be slightly out of time.
Easy couple of diagnosis things you can do though:
When its missing at idle, put on a good insulated glove, and pull the spark plug cables one at a time. If a cylinder is sparking properly, you should hear and see it arching as you pull the wire away... At this time the motor should slow way down and lag as if its about to die, as its just lost 1 cylinder. Quickly plug it back in after it does this. If you pull a plug wire and it doesn't change the way the motor idles, then you know that cylinder is what's causing the issue.. Check spark, fuel and compression to determine why that cyl is down.
It might help you to check your ignition timing. If you're getting a miss PLUS a backfire, it sounds like you might be slightly off. Put a timing light to it and see if you need to move your dizzy around.
Looks like you've got a timing chain motor? So chances are you didn't slip a tooth there unless you've done some work lately.
Just some place to start, if ya get more more specific info, I'll follow this thread and see if there's anything else I can think of.

Another thing I suggest outside of just diagnosis is to jump straight to a tune-up. Can't hurt.... and might help. 3 part seafoam the oil system, fuel system and intake system. Then do plugs (and wires if you haven't in a long time). Good flush and oil change and you might just fix that miss before you even knew what it was.

Has the ECU been rebuilt?

Stock ECU.. but I did notice that if smalls of fuel over a half a tank... wondering if the tank is going bad and maybe junk i the tank getting in the system.

Also... truck has a new chain.

Its more of a miss... I shouldn't have said backfire.

Does the ECU adjust the time and the dizzy is fixed?

Open the ECU case and check for burn marks, bulging capacitors and ooze on the circuit board. Also smell for a tangy electrical smell on the board.

The ECUs on the FI trucks are notorious for failing due to bad capacitors and can cause all kinds of drivability issues like the ones you're experiencing. There are several companies that can rebuild the ECU. Search around for recommendations if yours hasn't been remanufatured.

check the plug wires...for cracks and such as stated above...bad wires are a source of many "random" problems.

You mentioned you get a strong fuel smell when your tank is over half full. I had the same issue a couple of years ago. My fuel tank was rusting away and had some pinholes in it. I took it to a radiator repair shop and they sent it out for repair. Haven't had any problems with it since. If moisture builds up in the tank it will cause those misses you described when the water makes it to the engine.

I really should pull the tank and look.
I'm also gonna check the ECU.
I would love to convert this truck to Megasquirt and be rid of the factory ECU and give me the ability to tune with a laptop..