Shaking when accelerating, u joint or trans?

Whenever I get to 2000 RPMs it shakes pretty bad up under the seat, not the steering wheel or dash, and at around 70 mph if you touch the gas at all it shakes and vibrates loudly and when you let off it's completely silent, I thought I had a bad u-joint front end because I can see the bearing dust sprayed all up under the undercarriage and took it to the shop and the guys didn't think it was bad and thought it was the transmission and I keep seeing on here that people got a transmission flush and it fixed the shaking problem, what do you guys think? Also I keep reading that it's bad to flush instead of just refill, I have 264,000 MI and tow a lawn mower and trailor daily for my business,, any thoughts?

 

In my experience a U-Joint will be a very subtle vibration when accelerating. Not a violent shaking. Maybe not at 1st, but you should reach a point where you can hear a squeaking sound if you have the windows down and radio off.

 

Thanks! Yeah i heard a squeaking sound the other day, the shop thinks its fine and wants me to take it to a tranny shop and ive read on here that a flush fixed their problem but,, there's no play in the u joint so it seems ok but i see the stripe everyone talks about where a bad u joint will spray the grease and bearing dust up on the undercarriage and thats when u know he bearings are toast and he front one has that but