There is a piston that keeps you from going into reverse and Park at speed. It would only require taking the valve body off to check. 2nd to last resort. Last resort would be of course, to put a manny tranny in that bad boy

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg