2001 deville shifting problems

also if i do need to replace it can i use a older transmission from a 99 deville can I just take out the valve body out of the 2000 deville and put it in the 99 transmission so the connections would hook right up?

no check engine light no codes that point to a shift solenoid, I did change one of the shift solenoids because i had one laying around from a 99 deville but the other solenoid had a different connector I even changed the fluid the pan had a thick gray sludge on on it i changed the filters also becuase they had little flakes in it. when i took the shift solenoid out i applied 12 volt to them and they clicked. thanks for the suggestions just dont want to throw in another transmission if its not needed. I'll also try changing he vss sensor i think i have one laying around somewhere.

ok ill call the dealer up and pick them up, rather spend 100 bucks just to make sure before i swap the drivetrain out and spend 700 for a used tranny. The speedo works fine doesnt jump or stop working. When im cruising on the freeway and i let off the gas pedal the car's rpm will drop to about 900 it likes it falls out of gear maybe trying to shift to 4th gear but as soon as i press the gas it bumps into 3rd