big clunk....

[BIG CHEVY]

my transmission always made a metal type sounding clunk when going (for instance) drive to reverse.     this sound is multiplied by having it in 4 wheel drive. my question is not what is wrong......but how to diagnose.

can a u joint be bad even though they look and feel strong and tight?
because i initially think its not u joints. what would i look for in the rear end for fault?
also the BIG question i need answered is......can anything be wrong in the tranny(th350) making this lag?

description.....driveshaft turns about 1/2 inch to engage the gear of choice.
this action seems a tad violent.....for me.
and does not happen if u go from drive to park...then back to drive. only if u switch directions of driveshaft.
this truck drives fine and shifts perfectly in any mode. just clangs..

[choptop]

Does it look like either one of the pinion seals are leaking? I had one that did similar right before the pinion came apart.

[Irish_Alley]

i had one clunk while under a load or going from frd to reverse turned out to be a bad u joint that was in a 2wd. and also had one clunk due to slop in the rear

[Lt.Del]

check the trans mount rubber insulators.  I have had them go bad in my 91 sub and my 79 p/u.  Also check engine mounts, they will create play in drive line too, been there done that too.

Have someone put the truck in reverse then rev up w/ brake on while you look under hood at the engine to see if engine tilts any, then have the person put it in drive and rev it up while braking and see if engine tilts other way...do this a few times and you can see if there is play in the engine mounts.