engine vibration

I have a 355 chevy with a 4 speed in a home built 1940 Ford pick up street rod.  The problem is in 3rd and 4th gear when I speed up and then let off the gas I get a stong engine vibration.  If you push in the clutch the vibration will start to slow and then stop.  I do not get this vibration in the lower gears.  Here is some other general info:  The engine, tranny, drive shaft and 9" ford rear end are new or have been rebuilt.  The motor mounts are new. The engine is a chevy 355 chevy that has a comp cam HE274 cam.  It has a tunnel ram with a holley 675 street avenger carb on it. The raer end is a 3.55 9" Ford. The drive shaft is new and has been balanced.  
 Where should I start looking?  Any help would be great  

Possibly rear end pinion angle is off. Too much angle up or down can cause a vibration on exceleration or decel.  

is there a vibration when the engine is reved up and down or only when the truck is under power? what condition are the u joints in? drive shaft straight and phased correctly?  

Sounds like the drive shaft to me.  

I have experienced vibrations in a front end as well. After checking shaft and everything else, I talked to some friends and one told me to check my front wheel bearings. Sure enuff, I changed them and vibe went away.......FYI  

never heard that one before...........LOL YEARS ago my brother and I drove from CT to San Diego. He was in his Firebird, I was in my '56 Chev hardtop.We had switched cars and going thru Indianapolis his car developed a nasty shake in the front end. When I pulled over I discovered that some of the lugnuts were gone from the drivers wheel and the rest were lose. Seems that my 18 year old brother put regular nuts on rather than tapered lugnuts!