I have a 99 Jeep GC 4X4 Limited

You got a diagnosis from a guy who sells parts? You have problems in all different circuits including some that have nothing to do with the Engine Computer. Start by checking the charging system and measuring the battery voltage with the engine running.

A lot of code readers don't communicate on some vehicles. Many of the wires in the diagnostic connector are standardized but there are some that are reserved for each manufacturer to use as they wish.

The multitude of symptoms still suggests an electrical problem that is in common with all of them. The charging voltage is on the low end of acceptable but shouldn't cause a problem. You might look at the battery cable connections where the smaller positive wire bolts to the under-hood fuse box and the negative one bolts to the body. Look for loose nuts holding a large fuse in the under-hood fuse box.

Burned contacts in the ignition switch and overheated terminals in its connector can result in low voltage to the circuits they feed. You can often aggravate it and cause problems to occur by turning on additional loads on those same circuits. As an example, cassette player reversing problems can occur when you run the power windows down or turn on the heater fan on a higher speeds.

Loose rivets connecting the brass strips inside a fuse box have been known to cause intermittent problems. One way to find this kind of problem is to monitor the voltage feeding it. Use small jumper wires to connect a digital voltmeter to the radio's switched 12 volts wire, then see what happens to that voltage when the problem occurs. You can also tap into one of the switched wires at the ignition switch or one of the fuses. Once you find a point where the voltage drops, follow that back to the previous point and check there. Keep doing that until you find the bad connection.

The same thing can happen on ground wires except you'll be monitoring for a voltage that goes up higher than 0 volts. Often you will find ground wires from multiple circuits that are bolted to the body with a common screw, and they will all be affected at the same time.

The mileage is stored on the instrument cluster. Changing the other computers won't affect that. The mileage is stored in the Engine Computer too but not to run the odometer. That is simply to record how long ago a diagnostic fault code set. Even when we erase those codes, there is a permanent record that only Chrysler can retrieve. They can use that to prove or disprove a code had truly been set in the past.

Your odometer reading should not change when you replace any computer other than the instrument cluster. The cluster is the only one that needs to have the mileage supplied to the rebuilder so they can program it into a replacement before they ship it out.

In the early to mid '90s there was a "Maintenance Required" light that turned on at 60,000, 90,000, and 120,000 miles on vehicles with truck emissions systems. That included minivans. On those, once the light was turned off, the scanner would display the mileage stored in the Engine Computer, and ask if that was within 50 miles of the odometer reading. If it was not, you were simply asked to type in the current mileage to correct it. You and I will never have that option with the instrument cluster because that would be how the vehicle's mileage is illegally rolled back. Changing the mileage in the Engine Computer just corrects how long it will be before the "Maintenance Required" light turns on again.

You might be reading about the sad GM owners who run into problems that GM designed in to cost them money after the sale. Their cars need to have replacement computers programmed to the specific car by the dealer, otherwise they won't work. You can't buy a used computer from the salvage yard in many cases.