4th gear 2800--3K rpms

Hello Everyone

I just got over my 6 pack issues (thank you Brian) and today driving to the shop to have something looked at, I realized (on the highway) that when I shifted into 4th gear and the car reached around 2800 to 3000 rpm there was a wicked vibration on the shifter,car.... I let go of the gas back to 2500 no problem at all.
This is a new rebuilt engine (150 miles only). So the car is still going thru the break in process, it's not being abused (for now,lol ) but I never had this problem before.
Yes the front tire is off a little as I have new rims for the car with tires but haven't put them on yet as I am waiting for the front tires to arrive and then I'll do a wheel alignment one shot, new rims, new tires, etc.... But like I said I didn't have this problem last yr before the rebuild all of a sudden I have this problem.
The mechanic said I may need a driveshaft balancing.. First time I hear of this happening from one day to the next.
Any help will be appreciated.
Thanks.
MAPS
Might want to double check those universal joints while your under there.