vc transmission problems

Hi guys new member long time holden lover here.

I own a vc commodore 5L v8 auto trans, and the other day i drove it up the street and there was a loud clunk.
After the clunk the transmission wouldnt engage in d,2 or 1, yet would engange in R.
Ive check all my fluid levels ect and am using the correct fluids.
Ive aso checked the adjust selector linkage and everything seems to be in order there.
Any ideas what else i should look, before i start the long haul of removing the transmission or taking it to a mechanic

cheers
Counc