Where to begin...
Most of these items have been covered in great length on this forum, you should spend some quality time searching, especially in the tranny section. Brooklyn and Glacier are great resources.
Was the fluid level full at the time you were having that problem with the gears shifting? Low fluid often causes shifting issues, and they go away when fluid is added.
As for your leak, let's start with the basics: Are you seeing tranny fluid on the ground under the truck?
If not, it's probably the modulator. The fluid gets sucked into the engine and burns away, you get no outward indication (some people report more pinging than normal).
As mentioned, pull the vacuum line off the manifold tree (aka octopus). If you see ATF in the vacuum line, your modulator is bad. It is located on the pass. side, on the top half of the tranny. This means you can't see it from under the truck. There are two options: remove the cat, or go through the access panel on the hump in front of the center console. This is a time-consuming job, either way you do it!!!
If you see ATF on the ground in the front of the truck (near the radiator), your tranny cooler (rubber) lines are probably leaking. These are easy to replace, just be sure to pull the grille and clean the front of the A/C condensor (or radiator of you have no A/C). This will help your operating temp.
If you see ATF on the ground under the tranny, check the pan to see if any bolts are missing, or if the leak seems to be coming from the pan gasket. Replace the filter and gasket, bolts, etc. as needed.
In terms of tranny repairs, these fixes are extremely cheap, parts would be under $50 total to do all the jobs. If these fixes don't stop the leaking, you may be looking at big money to fix something internal.
As for the "old fluid" concern, all I can say is just change it. I went through this as well, and spent a foolish amount of time putting it off, reading all sorts of online info, asking mechanics, etc. Bottom line is, changing the fluid won't hurt anything.
The main reason mechanics don't want to get involved is that lots of people abuse the tranny fluid, and only change it when they start having trouble. They bring it to a shop, have the fluid changed, and sometimes the problem gets worse. The real cause is in all probability the fact that they ignored it for so long, and the tranny was on its way out anyway, but they blame the shop, because that was the "only thing that changed..."
As someone bluntly put it to me: what possible good do you really expect to gain from old, dirty, worn out oil of any kind?
Of course, you really should be sure to not just drop the pan and change the filter, but do the full flush (there are posts here on the technique to do this at home). Your tranny sounds like it's leaking like mine was (I like to think of it as "self-changing", yeah, that's the ticket...), so your fluid may be fairly clean and you may get away without the flush, but that's your call.
Hope this helps.
Mike
