I have come to the conclusion that "never" is likely the correct answer. Unfortunately, that is not much fun. I do know, as others have stated, that for the best deal getting a truck out of existing inventory toward month's end tends to make a big difference. With regard to depreciation, I suppose one advantage of having a diesel truck (I hope I read that correctly) over gas which costs more at purchase is it tends to bring more on the other end with sale. The 6.4L engine has had mixed reviews, but I had one and liked it. The depreciation curve on these type trucks tends to be quite flat after two-three years and even with higher mileage. Part of this is the market in which they exist. Folks looking for an HD level truck usually know that is what they want, regardless of economy and get it. The other group are the schoolboys that want a big, cool truck but do not have a particular need (I was once in the first group, now maybe have gravitated to the second--though I am not in school). That group tends to buy also, with less effect of outside economy. My point here is that a truck like yours, in good shape and well-maintained, will hold its value in my opinion fairly well over the next 20-30K miles, allowing you to decide what you want because they seem perpetually popular. I view it as being OK to look and even buy if you can get that "end of month" deal. I know that I was looking seriously at a new GMC about a month ago and my dealer called me three days from month's end cutting another $1500 off the price. I decided not to buy then, but I think this action shows the thinking expressed by many in this post.
John M