Non-A-body 46re q’s
How many miles are on it and has it been overhauled before? Best to check fault codes as was mentioned.
The electronics in these are very failure prone--that includes the output speed sensor, governor solenoid and the governor pressure sensor. The RE transmission makes it's own governor pressure so to speak with the electronics, so it all needs to be functioning correctly. These rely on clean voltage--battery, cables and terminals need to be spotless. It's best to road test with a scan tool to monitor road speed and governor psi.
That takes care of the electronic portion. Now to the mechanical portion. If it has a converter noise, debris/big things in the pan, 2nd gear band adjustment stud buried into the case or is just plain worn out, then it needs to come out to be overhauled. No bottle of trans tune will fix it.
You can get diesel overdrive parts for steel 5 or 6 pinion OD planetaries, and higher OD direct and OD brake clutch counts. Best to buy a complete 47RE or 48RE overdrive extension housing for this.