On the right side of the transmission at bottom is the transmission oil temp sensor and if you pull the plate off that it is in there is a strainer behind it that might be worth cleaning to see if that helps.  The oil has to be drained to pull the strainer out but otherwise it's just 2 bolts and a gasket. 

Those transmissions are pretty well designed so the powershift can't be in two gears at once, the pressure switches and electronics are there to protect for that very reason but if the right sealing ring on the transmission shaft was leaking I suppose it could be possible.  There are pressure ports on the transmisson valve body where you could check the transmission pressures, I think it takes a 7/16" o-ring fitting if I remember right but the next step would probably be checking the pressures to see just what is happening. 

If by chance it is the powershift and it needs rebuilt the good news is that it only takes about $1,500 in parts and if a guy knows what he's doing it's about a day and a half job to rebuild the powershift on them.