Diagnosing sounds over the internet is notoriously difficult, so it ultimately might be necessary to bring it to a transmission shop and pay them for a diagnostic.
Most of the significant RFE problems were ironed out by 2005 to my knowledge, they're pretty well behaved machines by the time yours was built but time takes its toll on everything. I remember that a couple failure points were the snap ring at the front pump assembly could fall off, there is a revised (heavier) snap ring available. Removal of the transmission is necessary to install if that is diagnosed. And the end plate for the valve body can bend if the retaining screws for it were torqued incorrectly during assembly, there is a revised end plate available to address that too (and it's included in the TransGo shift kits) . That can be diagnosed and replaced with the transmission in the vehicle, as the whole valve body can just come right out the bottom once the pan is removed.
If you're not comfortable working in transmissions I'd still advise just finding a shop and paying them for a diagnostic. Might set you back an hour of shop time or a half hour but catching a problem early is a lot less expensive than waiting for catastrophic failure even if that just means you get to trade in a driveable truck vs. trying to sell a "needs work bring a trailer" truck.
