There's a drain plug on the torque converter, so maybe your mechanic didn't know that. Slipping into gear can be low fluid, valve body problem, clutch pack, and I'm sure a few other things. Replacement of the clutch pack(s) requires transmission removal.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.