HP 8150 wrinkles paper only while duplexing

[Jazzmanics]

I'm working on a 8150dn that wrinkles the paper and sometimes jams while using tray 3 or 4 in duplex mode only. The following is what I have observed, tested and replaced.

Tested and observed the following:

Unit has 400K pages
All trays (1-4) print fine in simplex mode.
Trays 1 and 2 work fine in duplex mode.
Tray 3 and 4 front side image is aligned and correct.
Tray 3 back side image (duplex side only) is skewed about 1/16in.
Tray 4 back side image (duplex side only) is skewed about 1/8in.
Lower left corner has a 1/16in diagonal fold at final output.

Replaced and Cleaned the following:

Replaced fuser assy
Replaced duplexer assy
Checked and cleaned duplexer switchback area (left of and behind tray 3)
Replaced PIU
Removed, inspected and cleaned the reg assy.

I suspect the paper is hitting something between the PIU and the reg assy.
I cannot find anything in this area nor understand why tray 1 and 2 work fine.

Any and all help would be very much appreciated

[MichaelTech]

The lower base transport assy. on right side. Open, and inspect the rollers.
This has it's own PIU, and check the rollers inside tray4{2000sheet}
If only skew on the duplex, it may be a worn registration. Replace.