At the age of the vehicle, I would recommend sourcing a low mile unit and having a shop swap it in. Most of today's front wheel drive transmissions (and even many rear wheel) are not rebuilt, and are swapped as a unit. I would recommend taking to a Chevy dealer, pay for an hour diag, and have them pull, document, and print for you the diagnostic trouble codes from the Transmission control module (TCM). This will give you a place to start to see if a rebuild is even warranted. Number one sign of a toasted trans is burnt/ black fluid and vehicle won't pull it's own weight, neither of which seem to be evident in your case. The first guy was kind of right, 4th is still driven by the torque converter, what they mean is that the input and outputs are RPM matched under normal 4th gear operation, so to minimize parasitic loss from trying to move the output turbine with fluid thrown at it from the input impeller, the unit uses the torque converter clutch solenoid to divert transmission line pressure to the TCC and lock front and back halves together. Youtube yr/make/model torque converter clutch solenoid replacement. They're usually about $50, plus fresh fluid and gasket if your old tears on disassembly. Once all but 2 bolts out (loose not out, one on each corner on the side facing towards you) give the trans man a good swack with a rubber mallet/ dead blow. This should cause pan to break free from gasket, and end away from you will drop, allowing the trans fluid to dump. Finishing removing pan, (I usually go have dinner/ wait to next day if I can to let it drop dry so to not fight trans fluid all over me), then disconnect electrical connections at valve body, a few screws, a shift linkage, and should come right out. Swap solenoid, reverse procedure, put 3 qts trans fluid in (Dexron 6), and start, check fluid engine running in park. Drive around 15 min, recheck level and add if needed.