let me clarify -- the stock jack is rather flimsy and it is not ideal for even quick, emergency changes due to the base and the shaft. i am indeed replying to the topic of the thread when i say that it is weak. if you are forced to change a flat on the side of a busy road or something, you do not have much margin for error as far as jack placement and slope of the road upon which it will be set. therefore, if possible, it would be better to buy a sturdier jack unless you plan on just calling AAA.
for changing a tire, in emergencies, yes, Iman74, i am saying it's probably much safer to buy something to replace the stock jack.
ToyotaDoodz -- it was the shaft that started to bend. the reason i tested it in my garage was not to change a tire myself, but to see how it would hold up if you were on the road working in less than perfect conditions when it might need to be able to support things for more than a minute. in ideal conditions, you can change a tire quickly, obviously, and the jack is therefore only depended on for a minute or two. in all likelihood, however, if you're out on the road or off road on an incline or if it's nighttime or storming or whatever and is going to take more time than usual to change, then it is important that the jack be able to be under there without breaking for however long it takes you to change the tire in any condition. (often requiring more than a mere minute or two)
so, that's all i was saying - was that even though i didn't come to this conclusion through changing a tire, i came to it through testing to see how the jack would hold up in the case of a flat in less-than-ideal conditions where you don't have to worry about the jack giving out on you or bending or needing absolutely perfect positioning.
i now carry a jack identical to the one in my mom's suburban which has a larger base and thicker shaft.
if you want to stick with the stock jack, my recommendation is that you carry at least one jack stand so that once you jack up the truck, you can place the jack stand along the frame while you work to give the stock jack a rest.
(while you work = while you change the tire)