92 Yukon: Transmission now 'coasts' no braking

Hmmm.. My 92 Yukon (TH400 I believe) has developed a condition in which the transmission seems to run normally in all forward and reverse gears, with normal torque converter slip, EXCEPT there is no
'grab' or engine braking when getting off the throttle in forward.. It just 'coasts' as if it was in neutral.. no engine braking in any gear. The engine will idle down, and then when you get back on the gas, it grabs normally..
Anyone seen this one?? What could be happening here???
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