I had similar symptoms except the car would go in reverse perfectly but no forward movement unless I really revved it but even then it barely budged.
When I put it on jack stands it too would run and shift normally.
My B2 piston was perfect but perhaps the piston rod had fallen out which they do if not carefully reinstalled. On another try with the rod carefully installed, I blew the B2 cover seal when I put in drive without any forward movement.
I pretty much gave up on that transmission since it blew that cover a second time even with a new seal.
I found another tranny reasonably cheap and it's been fine so far.

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DJ


84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012