Drop the pan, pull off the 1-2 accumulator housing and blow out the piston with shop air. Then check the outer spring - it may be broken. While you're at it, take a flat blade screwdriver and carefully move the band back and forth along the drum to see how far it will go from front to back. Any more than about 3/16"-1/4" would indicate excessive clearance. Those two things will conspire to give a hyper aggressive 1-2 shift in a trans with a 1600-1800 stall converter (factory stall for most 4L60E-equipped vehicles).
Dont drive it like that for too long. Ive seen them come in with the band broken in two pieces and broken 1-2 accumulator spring.
