06 Sierra Denali Transmission Vibrations

I have a 06 GMC Sierra Denali AWD that had a new tranny installed 5000 miles ago by local dealer.A couple weeks ago while cruising rural roads 40-50 mph,I get a Vibration from what I think is the tranny.It only happens low rpms,1500 or so and in top gear.If I give it a little gas it goes away.but you can feel it through the hole truck,even my wife noticed too,and she thought it was washboard bumps in the road due to winter,which is a pretty good description I might add.They best way I can describe it would be to compare it to the feeling when a throw out bearing goes in a stantard vehicle like my 82 Mustang. It that same feeling when you let out the clutch and just as it enganges it shimmies a little bit,except in my truck it keeps happening when your at a constant low rpm in drive(or overdrive I think it is) .My truck is an automatic. Its also the same type of symptom I had in my 2000 Ford E350 when a coil pack was on its way out,just not as strong of a sensation as that.Kinda like a little bucking or something,but mat it and it goes away,but let of the gas and lug the motor and its back.I know its tough to explain somethings to others,but I tried my best.If anyone has any ideas it would be great or if you have any questions to help diagnose it,lets hear them. It would be easier if the check engine light would come on and throw a code.Thanks.

I have a 06 GMC Sierra Denali AWD that had a new tranny installed 5000 miles ago by local dealer.A couple weeks ago while cruising rural roads 40-50 mph,I get a Vibration from what I think is the tranny.It only happens low rpms,1500 or so and in top gear.If I give it a little gas it goes away.but you can feel it through the hole truck,even my wife noticed too,and she thought it was washboard bumps in the road due to winter,which is a pretty good description I might add.They best way I can describe it would be to compare it to the feeling when a throw out bearing goes in a stantard vehicle like my 82 Mustang. It that same feeling when you let out the clutch and just as it enganges it shimmies a little bit,except in my truck it keeps happening when your at a constant low rpm in drive(or overdrive I think it is) .My truck is an automatic. Its also the same type of symptom I had in my 2000 Ford E350 when a coil pack was on its way out,just not as strong of a sensation as that.Kinda like a little bucking or something,but mat it and it goes away,but let of the gas and lug the motor and its back.I know its tough to explain somethings to others,but I tried my best.If anyone has any ideas it would be great or if you have any questions to help diagnose it,lets hear them. It would be easier if the check engine light would come on and throw a code.Thanks.
That happened to me.. I hit something and my driveshaft was out of balance. Got it fixed and that was it. Hope this helps.