My buddy would probably be able to point at the issue but I can't... my 4R70W was DESTROYED when I got my '00 Explorer, seemed ok when I bought it but after a test drive after I fixed the rear axle (stripped spiders) it had all forward gears, no neutrals or reverse... like $900 in PARTS later it was working, 20k later the only part we didn't replace (planetary assembly) blew up and it lost neutrals and reverse again... I think he found a good used one for $200 and we got it back together again... The 4R70W is a good trans assuming all the hard parts are in good shape.
That said, trouble shifting like that it's either the shift servo or something in the valve body unless the clutches are gone from my guess. The servo is like $8 on amazon so might as well do that if you have the valve body off...