This is funny, but I am in the same boat right now. I just left my local shop being told I need a new rear main and water pump (both are leaking). Mine is a 2001 Yukon XL with 263K and the original motor and trans, new transfer case at 130K. This is also my daily driver and I travel quite a bit. It is also in great shape and I plan on keeping it till it rots!
Local shop is going to do rear main, cam timing sensor seal, oil pan and water pump for around $950. I was also wondering if I should swap the tranny while its out. It seems to shift fine and I have always maintained it swapping the fluid about every 40 to 50K. As you posted, I want this one and done, not having to think about it in 6 months.
I know of a salvage yard in Marion, NC called J&T and they specialize in these vehicles so I can get a complete tranny out of a wrecked Yukon for around $700. Now you have me thinking about getting the Jasper instead since a wrecked one would be a little risky.
Ezdaar posted that we should replace it with the 4L80 and never worry about it again. Not sure what is in mine now, but why are they better?