had a rebuilt tranny installed , several months later we replaced the radiator shortly after that on christmas eve we blew the tranny collant line doin 70 down the interstate lost a lot of fluid pulled it home filled it back up it still ran, on new years in the same place it quit runnin we pulled it home AGAIN but it wouldn't shift after runnin a minute or two, so we bought another car and it sat in the garage our old s-10 finally bit the dust this weekend so i got on the search and discovered that our symptoms could be from a clogged filter and dirty fluid so before we dropped it and either rebuilt or got a new one we decided to change both and see sure enough the pan and filter where awful dirty FULL of metal which i'm guessing isn't a very good sign we put the pan back on and filled it adding one bottle of lucas and now it shifts into reverse and and drive but the rpms just climb when ya get going so wonder if the shift solenoids should be replaced ....thanks for all the great info on here it has saved us ALOT of money

 

The metal shavings in the filter = Toast!
Sorry to say but that is never a good sign. Sure the Lucas mucas might help a bit for now, but it's only a bandage.
Having the truck & trans run for a bit, especially at highway speeds meens the torque converter (essentially a pump) is pumping fluid out at a horrific rate. It'll only take a minute to empty the trans of fluid. That'll lead to the shavings you found in the filter.

Sounds like the seals are gone & BTW dropping the pan on the trans & refilling it only replaces about a third of the fluid. The converter holds about 2/3 in it. To get that removed you have to take it in to get the trans flushed out. But I doubt that will fix the shifting issue.
Just my thoughts having had to get my trans rebuilt because of blown seals - weak reverse & a long hang time between 1st - 2nd. Lucas helped it, but it still wasn't getting better after awhile.

 

+1, had similar issues about 3 months ago. 1300 dollars later I have a rebuilt transmission lol