Hi, I have a 1990 Si cable tranny (L3) with 240k miles on it. All the gears go in very smooth, there is no grinding at all at high rpm either. However here are my issues:

1-On light acceleration, deceleration and constant speed I can hear it whine, especially in 3rd and 4th gear at low rpm. I dropped the transmission fluid and the level was low, I dropped around 1-1.25 litres and the fluid was very black. After replacing it with new MTF and a bit of lucas additive I noticed no difference at all.

2-If I grab both axles where they go inside the diff, there is play. I can move both axles up and down and side to side maybe 1/16th''.

3-This is hard to explain, but when I am at a dead stop and let go off the clutch, I feel there is a lag in the transmission, then I hear a small knock (and also feel it in my shifter) and then car moves foward. This is very minimal but I feel it. Same feeling at loose engine mounts but it's not the case. Kind of like if the transmission does 1/4 of a turn before it starts driving the wheels.

Are all these issues related to my differential? If not what could it be and would it be worth it to repair or just find another transmission? Si cable transmissions are pretty hard to find around here.

I have found a 96-00 s40 EX (canadian Si) differential and input shaft and 4.4 final drive for 60$, is this a direct replacement and will it fix my problems?

Any input apreciated, thanks.