I have a 98 with 185K...and have the same problem. My mechanic who has always shot me straight said that with high miles most likely it is the timing chain. That was at 142K... I have stopped trailing my boat to far (5000 lbs). Hope this helps.
If only under load then......spark plug leads.
sounds like a cranksenser.it will put your timing out when your under load .markstang
A flashing check engine light is usually a misfire.
heavy or strong misfire, or a bad MAP sensor.
The LS is prone to coil failure which leads to a misfire in the affected cylinder. A misfire due to a failing coil usually happens when the engine is warm and under load (low RPMs, accelerating or going uphill). The misfire may feel like a transmission problem because it causes the car to buck and stutter as though the gears were skipping or getting stuck briefly. The worse the misfire gets, the harder it will be to accellerate. Sometimes the misfire condition can be overcome by applying more RPMs or reducing the load on the engine. See my post about the valve cover leaking issues contributing to this issue.
exhaust leak. gaskets around the manifold. probly has a misfire at idle to i bet?
chk. compression,150 is normal,about 120 or less in no. 6 and it will miss at idle,but you wont feel it under a load.
From most to least likely but not a complete list:Drive shaft or half shaft trouble, bad u joint or a worn CV joint will vibrate under load.Colapsed or damaged motor mountEngine run problem, like a misfire
You probably have a misfire, but it doesn't show up if the engine isn't under load. When you put it in gear the load causes the misfire to show up and you can feel it. Run a computer diagnostic test and a compression test. You can borrow, rent or buy a computer diagnostic scanner for your vehicle at most auto parts retailers; often they'll even show you how to use it if they have the time.
When under a load the errors will be shown faster
incorrect fule mixture to that cyllinder, rough idle, car can sputter when under load, underperformance, some/all of the above. it negatively will affect proper air/fuel mixteur causing the cylinder to misfire
You solved the problem yourself. DO NOT USE overdrive unless you are on an open road and driving over 55 miles an hour. If you do you could cause the transmission to start slipping and pay thousands to repair expectially if you are hualing a load.