The forward clutch pack is not applying. Look at the sealing rings on the stator support, the belleville spring and the apply piston seal on the forward clutch. (inner and outer).
If you don't find any problems there then look at the valve body.
Beyond that it may be broken sun gear or planetaries. As a last result look for a cracked case - I have seen a couple that broke from too much force applied by the intermediate servo. It usually happens with a large servo that has been modified and high pressure working it. Like a manual shift valve body.
Let us know what you find.

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Paul
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retired mechanic after 35 years
specialized in Holley carbs and Ford Automatic Transmissions