If the motor's good, the thing don't squeak, creak, or groan,.....and its a work truck,....throw a rebuilt tranny in it, and go. Hell the main bearings could fail tomorrow, but damn,... 393,000 miles?

It's a "run it till it dies truck"....It owes you nothing. If it runs good, let it,....if you have concerns, dump it, and get another....