Welcome to the forums, Alberto! Your photos are very helpful, and so is your description of the problems...Here are some observations:
1) The floor getting hot and the high altitude lack of performance are related to the catalytic converter and exhaust system. This engine uses a Pulse Air catalytic converter, and the air tube from the exhaust head pipe to the catalytic converter has been eliminated. This prevents complete burn in the catalytic converter and also eliminates the pulse air system's cooling effect at the converter. (Lack of a Pulse Air hookup would fail a U.S. emissions test.) In your case, the catalytic converter, if still intact, is either not functioning properly or is likely getting extremely hot from a rich fuel mixture and the missing Pulse Air function. (Tip: The Pulse Air catalytic converter should have an air inlet pipe that works with the pulse tube system.)
2) You have valid concern about the exhaust heat and the plastic fuel tank. In many cases, the heat you describe melts the fuel tank. In a worst case scenario, this could cause fuel to leak onto a super hot exhaust pipe and either set the vehicle on fire or cause the tank to explode. From the tailpipe's "rich" burn signs (black and sooty at the exit), plus your poor fuel mileage, it's quite possible that the carburetor is running way too rich. This could be either a carburetor issue or the wide open throttle/limp mode may be operating all the time due to a wiring or vacuum circuit defect or tampering.
3) Click on this link for the detailed 4.2L BBD carburetor rebuild articles at the magazine. The BBD carburetor requires careful attention to detail during a rebuild, Alberto. You will also find a number of articles related to 4.2L engine tune and troubleshooting at the Search box keyword "BBD". To save you time, here is a link to the lengthy list at the magazine: http://www.4wdmechan...rch-results.php
4) The previous owner of your Jeep Wrangler apparently disconnected and eliminated a number of vacuum hose and coolant features. Here are the vacuum circuit diagrams for your engine, you can compare what you have with these PDF diagrams: 1) for the 1989 4.2L Jeep Wrangler with a manual transmission
YJ Wrangler 4.2L Vacuum Diagram.pdf 554.59KB
7 downloads and 2)
1989 Wrangler 4.2L Engine Vacuum Auto Trans.pdf 40.47KB
10 downloads for your automatic transmission equipped model.
Note: The emission and carburetor circuits are closely linked together on this 4.2L engine. The ignition is somewhat independent but also works through vacuum, coolant temp and ECU timing controls. This is a complex system, the reason many owners convert to EFI. Howell, Mopar and MSD have EFI systems as you will see in the article on the MSD Atomic EFI conversion for a 4.2L. (Click on the link—there are five detailed pages to this article!) Howell is the easiest system to install and cost effective if you want to do a conversion to EFI. You can restore the BBD carburetor, vacuum circuits, ECU circuits and such; however, it will take time and patience to sort out all of the modifications and tampering that the previous owner has done to this engine's vacuum, emissions and coolant circuits.
5) The rear diff cover looks normal, it does seem like a low fill plug, but that's the Dana 35 design. The noise and hop sound like a factory limited slip differential that's either defective or in need of friction modifier and an oil change. Remove the diff cover to drain, do a thorough inspection, and if there is a factory multi-plate Spicer limited slip, there should be a tag specifying use of special lube or lube plus friction modifier. Friction modifier breaks down, and fresh oil and modifier often eliminates clutch plate chatter and wheel hopping.
6) Your engine's fuel filter should have three attachment points and fuel lines. The third line returns excess fuel pump volume to the fuel tank. This serves two vital functions: 1) reducing load on the carburetor needle and seat and 2) lowering the risk of fuel vapor lock by keeping fuel moving and not stagnating in hot lines at the engine. Vapor lock is a common phenomenon on mechanical fuel pump systems without a third pipe (fuel return) to the tank. High altitude driving and hot under hood or exhaust temperatures increase the risk of fuel vapor lock. You need to restore the 3-line system, the engine could have been experiencing vapor lock at higher altitudes under extreme load.
Worth noting, poor engine performance at 3000 meters is also a function of altitude. A naturally aspirated (no supercharger or turbocharger) engine loses approximately 3-5% of its horsepower per 1000 feet of elevation. You're talking about 9,000-plus feet above sea level, so that could be as much horsepower loss as 45%! This would create a very rich burn condition on a carbureted engine, too, since the carburetor has fixed jet sizing. There is a primitive "wide open throttle" (WOT) fuel enrichment mode, which is not intended to compensate for altitude.
You have an oxygen sensor on the 4.2L engine, designed to help regulate fuel enrichment (to a very limited degree) and spark timing, primarily for tailpipe emissions purposes. It does not change the air/fuel ratio constantly like an oxygen sensor helps to do on an EFI or MPI system with electronic fuel injection. In the U.S., improved high altitude operation is another reason why 4.2L Jeep owners convert from the BBD carburetor to an EFI system. EFI will instantly compensate for altitude changes and air/fuel ratio demands.
This should get you started on troubleshooting and evaluating your Jeep's exhaust system, fuel circuit, vacuum circuits and carburetor. Ask questions as needed, we can take this further.
Trust this helps, Alberto...