Took a couple of trips up to northern AZ the past month and I have this pretty significant vibration in the drivetrain when coasting down hills (letting off the throttle). Once I apply the throttle, it goes away. U joints have never been replaced but lubed religiously. 150k miles with 4" springs and 2" shackles. I have shims between the springs and the perch. Easy way to tell if it's the U-joints or the pinion bearing going south?

'86 4x4 SR5 22re 5" lift 35" BFG's and 5:29 gears.
'05 4x4 DC Tacoma TRD Sport