Honestly i wouldn't tow more then 3k with a xj because of its short wheel base.When it comes to towing the longer then vehicle the better.

 

I have a 4x6 Lowes utility trailer that I once loaded up with too much stuff and probably was close to 2000 lbs total for trailer and contents. Two things to note about this setup

1) Had much difficulty climbing mountains through the Poconos in PA and on I66 in northern Maryland. Had to down shift to 4th and sometimes 3rd gear to complete the climb but it did track Ok even in wind.

2) That Lowes trailer had no brakes and I wrecked the XJ when someone made a stupid panic stop in front of me. Without the trailer it would have been a no problem hard stop but that trailer pushed me right into him and then some.

Hard lesson.

 

The problem you mention not be able to stop and being pushed forward also exist with the Wrangler, CJ, LJ, etc.

Even though the XJ tow capacity is rated at 2000 and 5000 lbs maximum. I don't think I would want to tow very often with the XJ at around ~2000 lbs without trailer or toad brakes. My state has a GW of 3,000 lbs regulation. I towed once @ 2200 lbs, 6x12 TA cargo trailer with brakes disconnected. The load wouldn't stop very quickly, I needed to compensate for the GTW when towing and stopping (would not be able to perform an emergency stop).

Though in some states 2000 lbs is under the State and local jurisdiction trailer towing law(s). The Cherokee brakes alone may have a hard time stopping even if the brakes are in top condition.

Braking in wet conditions with or without a trailer can sometimes become problematic, depending on the amount of road water, the XJ has a difficult time stopping when not towing. Other limiting factors may include amount of road dust, mud, snow, ice, etc.
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Trailer and toad brakes are mandatory in many states depending on the GTW (gross trailer weight). Some states have towing laws that doesn't apply to trailer weight but to stopping distance of the tow vehicle with a connected trailer. Some states have lower weight limits for "new" trailers and semitrailers. Some states include clauses such as e.g. that the GTW cannot exceed a % of the GVW.

Digest of motor laws: http://drivinglaws.aaa.com/tag/trailer-brakes/

When driving without a trailer in large puddle of water the Jeep not going to stop very quickly. Same is true for snow and ice and some of the other types of road conditions. The GVW of a trailer or toad increases stopping distance. Having trailer or toad brakes significantly helps to improve stopping when on and off road, under various road conditions.

 

I've done quite a bit of towing with my XJ, and I'd think a horse trailer would be a bit much. It pulls a 3000 lb utility trailer with a couple of ATVs or some lumber just fine, but my dad's 4000+ lb camper was another story. The trailer weight was enough to bottom out the suspension. Also, our beloved straight 6 really had to work to pull the thing up hills. It did it, but it drank dead dinos like they were going out of style to do it. A year later, we took the same trip using my dad's F-150 to pull the camper and the XJ bob tailing behind, and actually used less fuel between the two vehicles than we did the previous year with just the XJ.

Pulling horse trailers is really what the Ford Excursion was designed for.