Transmission shifting fast into overdrive

I ran over a 18 wheeler tire , it knocked my exhaust a loose & a few days later my
transmission started shifting into overdrive at 30 mph. Can anyone tell me what they think has happened? Thanks Job1972

What year is the car?

Gm. 93 Chevy lumina euro

Over the net, no way to pinpoint the problem, get it up in the air look for any related damage, connectors, dented pan for example.

Ok. Thanks.