There is a few things you can check -
Pressure loss at operating temp could be a worn charge pump - was this replaced when the converter was rebuilt? No it wasn’t replaced
What did the central hub & bushing look like in the converter? Was the bushing replaced? Hub worn on one side? Bushing was replaced , the hub all bearings and seals were replaced ring and gear were replaced
Next, there could be broken oil control/hook rings on the transmission shafts. When these fail oil is sent to two different clutch packs when a track range is selected
ie - RH low is selected & some pressure is sent to RH high.
What this does is cause the packs to fight against each other while operating - like driving with a brake on.
It usually causes transmission overheating & damages the clutch plates in the packs.
If you click through each individual gear - at operating temp & at an idle - & watch the transmission pressure gauge & take notice of how long & how strong the engaging 'clunk' is, you will get an indication of where the leakage is occurring.
A good transmission will have nearly the same pressure in neutral & when engaged & each range change will feel the same & take exactly the same amount of time.
Forward/Reverse oil control rings are at the rear cover PTO housing.
LH & RH High/Low are up high on the front cover - you need to remove the brake housings to inspect them.......
If the oil control rings are broken - then it's likely the clutch packs are damaged ..... requiring a transmission overhaul to repair.