Vibration while stopped with shifter in 3rd, possibly torque converter lockup?

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Actually 1983 K5 Blazer. 300,000 miles I'd guess, 6.2l diesel, 700r4 auto transmission, Motor is military take out and Trans was rebuilt some 12 years ago by the place I purchased it from. drain and fill twice in last 4 years on transmission fluid with one quart of Lucas fluid conditioner added, recent oil change, tv cable adjusted today after repair on throttle cable connector at db2 injection pump due to excess play. The vibration has been ongoing for a few years so I replaced the TC lockup wire connector at trans due to bare connector wires touching earlier this Summer, and replaced the brake light switch also. The vibration is present after a short drive when I am at a stop light with the brake peddle depressed, but if I momentarily shift to neutral then back to 3rd the vibration goes away. A mechanic adjusted the tv cable this last summer and stated that he thought the tc is staying locked up, and also recommended a new throttle valve cable. Any thoughts on the cause and solution would be appreciated!