1977 c20 chevy /350 turbo



Hi : I'm having transmission problems, my 350 turbo will not shift out of low gear,, it has reverse ,park , low gear, low does not slip,, when I take off with the selector in drive ,,the trans stays in low until I let off the gas ,, then it seems to be in no gear at all,,revs up, but if I push down on the gas it goes right back to low gear ,or,,when I drop the selector in low it shifts back into low,, can anyone here give me some idea ?? is my trans ruined?? oh by the way this started after my wife took the truck out and kicked the 4-barrel in ,, guess that is what I get for having great sounding exhaust,,,, she then came to a stop sign,,after that,, low gear only??,,,, I did check the kickdown cable & modulator vacuum ,fluid level, those things seem OK?? I'm not sure how to check the Governor to see if it's OK,,,Help!!!!!!