89 chevy vibrates at low speeds

[ru469]

1989 chevy 1/2 4x4, 350 engine, 700r4 trans., transmission was rebuilt 3 months ago, new tires over a month ago, new front shocks a couple of weeks ago, now just a couple days ago, the whole truck vibrates/shakes when i go slow, especially when i first start it and start driving, it gets better the more i drive it to where you barely notice it, if i throw it in nuetral while going slow it goes away, if it is doing it at say 20+ mph and i throw it into 2nd gear the engine revs up but the vibration goes away, it shakes the whole truck not just the steering wheel, its weird that it gets better as i drive it, when i first take off after starting it, it feels terrible, i don't know how to check if it is in the wheel bearing or transmission, i also had 1 u joint replaced when the tranmission was replaced, any help is appreciated, thanks