'03 Toyota Tacoma auto transmission

Q: Which is better, auto transmission 'drain & fill' or a transmission flush?

Details: I am not original truck owner - don't have car's history - don't know if abused. Noticed tow hitch ball scored and rusted. Recently (after approx 5k miles) transmission has been slow to shift to higher gear.

Took to local dealer who pulled pan and saw debris; believe solenoid is clogged. Wants to install a rebuilt auto trans.

Don't have cash yet to replace trans ($3,500)and want to extend life of current unit.

Tacoma forums recommend transmission flush over drain & fill, but I'm afraid flush could drive any debris further into solenoid or shift circuits.

Which method is better, or at all?