First- get back here safe and thank you for your service.
In regards to what you want to fix before you get back- Don't. Just my opinion and everybody has one but why fix something that isn't broken or isn't giving you problems? You are opening the door to creating problems. One bolt gets broken during disassembly and a "feel-good" job turns into a nightmare.
If it was a timing BELT I'd change it. I have 202K on mine and unless I pulled the timing cover for something else and saw the chain was sloppy I would not f*ck with it. If it gets loose, you'll know it (noisy and eventually even wear through the timing cover if it gets bad enough). If the chain is worn out them the cam probably is too. If the cam is flat the rings are probably gone.... where do you stop? Just my opinion but leave it alone until you have to do it.
As far as the tranny- if it is running/shifting fine without any weird behavior or slippage LEAVE IT ALONE. Drop the pan and change the fluid and filter- its best to remove the pan (you'll probably never get the drain plug out if its never been removed) and let it drip all night (assuming you can park the truck indoors in a clean environment). Cut/smash the old plastic filter housing and see what the filter element inside looks like- it should only have some clutch material and a very small amount of metal, if that. The magnet on the bottom of the pan should be pretty clean too- mine just had a very minor amount of sludge. Put in a new filter, new fluid and let it go. If the filter or pan has a lot of stuff in it, you might have a problem coming. Do NOT power flush. Just drain and refill- several times if you want to really do it right. If you are worried about the tranny maybe bank some money for a rebuild for it and when it fails. You might get another 100K out of it. It might outlive the rest of the truck. Mine was in very good shape and it looked like it was the original filter and fluid. I was worried about the condition of the tranny until I saw that.
Save your money and do the other PM stuff first- someone said the rear end (I did mine), brake fluid, coolant, I even exchanged my power steering fluid. With a truck of this mileage, something is going to go sooner or later so save your money for that instead of just fixing stuff before it breaks. I'm guessing that rear main seal will be first if it hasn't been done yet (trust me, I just did mine)
