99 Suburban towing problems

I own a 99 Suburban 2wd with 113,000 miles. When I tow the smallest trailer I get a bad shudder in the front end and it feels like it is losing power going up hill. I have bought new tires and it still shudders. I have been told it may be the Torque Converter. It occurs in O/D or 3rd gear. The owners manual says tow in O/D. The vehicle does not shudder without trailer. I bought the vehicle when it was 1 year old and it has had a short whine noise from the rear end at approx. 30 and 50 mph under strain with or without a trailer. I added a small bottle of fluid at the advise of a mechanic ( The name escapes me)made by Ford into the transmission. The truck started skipping very bad and I had to get the transmission flushed, The skipping stopped and I'm back to my old problem. One time towing a small boat uphill it shudderd and the check engine light came on and went out. I have not had that code read. It was several months ago. I believe all of this is related. Help Please!

Could be a misfire happening under load or it could be the tranny. A good scanner on it while the problem is happening would help diagnose the problem as engine vs tranny.

//2000CAYukon

Thank you I will try that. Went to Auto Zone and had the code read it was P0430. Lights been off a while

PO430=Low Cat Efficiency Bank 2 (pass side).

Before you replace the CAT make sure the following are not present.
1. Misfire!!!
2. High oil or coolant consumption.
3. Retarded spark timing or weak spark.

So, it seems to me that a misfire may be causing your problem and the misfire is most likely on the pass side. Need to pull all the plugs and have a look at them. They should all have a light tan color to them. This is a very basic thing to do to determine which cylinder is not performing properly.

Let us know.

Ok thanks I will check the spark plugs. It had a coolant leak about a year ago, dripping behind the motor and I had the Intake Valve Gasket replaced. But had to add a little in the reservoir the other day.

Thanks to all who responded to post. I pulled the plugs on the passenger side and found one not gapped correctly. I pulled my trailer to mechanic and it test drove fine. I'm not sure why but I believe this fixed it.