2002 TB: Engages into 4-hi&lo by itself

[BigRed8100]

At 38,000 miles, the Sun Shell (spline or whatever drum?) went out in the tranny taking out all gears except 1st. Now 100 miles after this was fixed, the Trailblazer shifted into 4-Hi by itself at highway speed (70 mph) with the 4-lo light flashing. My wife mentioned it slowed the vehicle down drastically - as if she was going to end up on the hood. Anybody else experience this - could it be related to the tranny fixit ?