'70 142 lowered 1.5"...stock M40 wobble stick, 15' wheels, 60 series tires

Horrible screeching scrunching scrutching sound on decel in 3rd and 4th, not in 1st and 2nd, sometimes while feathering in cruise, doesn't always go away when you reapply throttle...seems unrelated to engine speed, comes from under the car...not really from driveshaft area, just behind front seats sorta

Spider gears or pinion bearing?

Also terrific driveline vibration over 4000, at least I think its driveline, seems to come from transmission, but I had it rebuilt with new main brngs and seals...driveline was balanced, but center bearing looked OK, so was reused.

Could any of this be pilot bearing...I put in what was recommended a little NSK 201RR ball bearing?

Where do I spend money first?

Ron

In addition to what's already been suggested, take a close look at *all* fasteners for looseness, starting at the bell housing and working your way aft.

And I gotta ask... was all the work mentioned in your first post done at once? If so, did this noise start immediately after reassembly of the parts to the car? Did the gearbox rebuilder *really* know what he was doing? For reasons unknown, I've had an M40 rebuild screwed up beyond belief (wouldn't even shift from one gear to another) by what was supposed to be a reputable manual gearbox specialist... admittedly, not a Volvo-specific rebuilder. I now do my own. :-)

WRT pilot bearings... I don't recognized the number, not that I know them all. In any case, whose recommendation/listing was that?
--

Gary L - 142E ITB race car, 73 1800ES
BlueBrick Racing Website
YouTube Racing Videos

It COULD be the pilot bearing but I sorta doubt it.
Are you sure you put the snap ring in behind the pilot bearing?
It is kinda odd that it would do it in 3rd and 4th but not in first and second.
I would first put it up on ramps and check the driveline carefully, noticing any
looseness in the U-joints and the center bearing area. Try to move the driveshaft.
If you can move it far enough to touch the floorboard/tunnel anywhere that might be part
of the problem.
Are you sure your front and center U-joints are properly aligned?
Here's a pic of how they should be:
(Sorry, I can only get to the first 20 of my 34 posted pix and I really don't
feel like going through 2000 pix to find it. Push Jarrod to get a decent search function!)

Check your motor mounts out rael good. just a thought.