Drivetrain must be cpu related

[pontiacman469]

So my 99 5.7/4l60 has been clunking hard in the morning,not everyday blamed it on bad u-joint last year i replaced 1 front and 1 back joint others didnt need it so question it being that.Tonight i open door to put window down checking headlights after turning key on to put said window down about 5 secs later heard said clunk.Huh truck not running,not in it pressing on pedal etc.Tells me must be a solenoid or computer function behind this?Any input where to look is appreciated.Thank you -Andrew