intermittent driveline vibration in '96 K1500 Suburban

Recently, I've noticed what feels and sounds like driving over finely grooved pavement. It's intermittent, and for a couple of weeks, I thought it was simply small perpendicular pavement grooves like one occasionally
sees on major highways. Turns out, it's not. When it starts, I can stop it by letting off the throttle, and it's more evident when towing and/or in "overdrive" mode. I recently had the rear axle seals and brakes replaced by a GM dealer, and I don't remember this happening before that. Any ideas what this might be?? Thanks Guy
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I'd look at the driveshaft u joints. Also did the dealer take the shaft during this repair? If so(shouldn't need to), may not be indexed properly, possibly they left a rear wheel loose?

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The first thing I checked when I realized that it wasn't the pavement, were the rear wheel lugnuts. All ok. I don't know if they took the driveshaft loose or not, but I'll check with them this week. Guy
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Update... I took it in to the dealer about 8 months ago, and they said it's the transmission. They have seen it before, and said that replacing some "harness" in the transmission "might" take care of it, but the worst case scenario was a new transmission. They replaced the harness, and it reduced the problem by maybe 95%. I parked the vehicle until last month when I got it out again, and the problem has now returned as bad as ever. It seems crazy that the whole transmission would have to be replaced to fix this! Anyone else experience this problem and maybe have a better solution? Thanks
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Never heard of this, is there a tsb explaining or relating to this problem?

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Still dealing with this problem, and it's getting worse. It seems almost like the lockup torque converter, which I assume this has, is chattering when trying to engage. If I give it more throttle, it stops, and if I stay in third and not D, it's not near as bad. Any ideas?
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