Won't shift to Overdrive ✓engine light on

Okay so my check engine light came on, and I noticed she not be shifting into overdrive, as well as a hard shift down to 3rd. For awhile it was hard shift to 4th goin uphill, then a REAL HARD jump and shift back to 5th...
My rear diff needs changed but my drain plug is stripped...I cant get the damn thing off!

 

What code(s) is it setting?

 

When you say rear diff needs changed.
Do you mean the fluid?
Or the actual differential/3 member?

The key to the Diff Drain and Fill bolts are a 6-point socket.
Those bolt/plug heads are soft.

 

Okay so my diff needed fluid, so I was told 3 months ago, and I was just gonna add to it but I don't wanna mess up that bolt either.
And I haven't put it on a code reader yet.

 

What was the outcome of the overdrive issue if you don't mind me asking?