99 Durango Overheats when towing

I have a 99 Durango with the 5.9l motor. I moved from Phoenix to Colorado with a trailer and had no problems last fall. Since then, I had the heater core and thermostat replaced since the heater didn't get very hot. The transmission went out so it was rebuilt also. This spring I towed a trailer and the Durango overheated. About 230-250 degrees. Back to the radiator shop for a complete flush and power washout of the radiator. The Durango still runs very hot when towing. I overheated with a 4500 lb. load and again with a 1000 lb. load. I am not talking much weight. Only occasionaly does it run warm with out a trailer. Sometimes reaching 200 or a pinch over. Why has the overheating only started since the heater core and transmission been worked on? Does the system need to be bled? Has anyone had this problem. I would love to tow again but am considering selling the Durango when it's cool this fall versus spending hundreds at the shop troubleshooting this. I would love to hear from the guys who have actually fixed their overheating problems with Durangos. Thanks.

I took a shot at the water pump and that seems to have fixed the problem. I could not find anything visually wrong with the old water pump even when it was off. I still run warm occasionally but the temp comes back into the normal range on downhills and during times when it's not pulling very hard. So, for know, I will consider it fixed.

Try a bottle of Water Wetter (available at Autozone, Advanced Auto, or even Wal-mart) I was skeptical, but it really did help! I've even added to my quad anow the fan doesn't kick on near as much. You could also try a 160 thermostat. Durangos come with a 190 to get the catalyst to work and pass for lower emissions. Great for emissions, bad for engine life.

I would suggest against the 160 T-stat.
If the engine stays under 170 for an extended amount of time the Check Engine Light will come on. If you want to go to a lower T-stat, you can use a 180 without throwing a CEL.
It sounds like you have a cooling system problem of some sort b/c you shouldn't be having overheating issues at all if everything is working properly. Make sure you have a competent mechanic really take a look at what is going on with the engine to cause the overheat condition.

Haven't had my CEL come on about 6 months. How long do you think it might take? or is the engine running above 160 all the time anyway?