2002 4l80e no reverse

Picked up an 02 gmc 1500hd 6.0L 4l80e no reverse, thought i could put the pin extender on and call it a day, still no reverse. Deciding whether to rebuild or get a junkyard trans, gonna check the junkyard for compatible while i'm out tomorrow, there's a 2005 silverado in their yard, along with an 2007. i know the 07 is incompatible if it's a 2007.5 with the displacement on demand and the trans got updated with a TCM. the 2004 and up got an updated valve body, wondering if that's still compatible with my 2002, or if i can swap my valve body onto the 05 trans to make it compatible. Also looking for any advice on diagnose/repair of the trans in the truck, i greatly appreciate anyone'stime in advance. I didn't manage to test 1st gear overrrun engine braking, kinda assumed the reverse servo rod length had to be the issue since there is so much about it on the web, my rod with the extender measures 3.817" and in the trans with hand pressure i measured 0.120" of throw, i know that's supposed to be down around 0.08" but I dunno how to get down to that, I don't have a spare servo cover to weld a nut onto. wondering if a new rod plus the extender would make the rod long enough to bring that down, but then the rod would be out of spec, i think spec is 1.6-1.8" and i'm just a hair over that already. Maybe throw is so long because the tab is broken on the reverse band, or a holding bolt is sheared, maybe my band is burned, trans doesn't slip in reverse but completely freewheels. I"m gonna unplug the electrical harness and see if the above 8 mph safety do not engage reverse is the issue before i drop the trans for a replace/rebuild. I found sonnax page about splitting the circuits for diagnosis to be very informative, but all this reverse diagnostic is so complicated and the jargon is hard to not get lost in. Paid $1000 which is a huge steal for a 4x4 quad cab in utah even if it is a painters truck with paint on the seats and bed, but this experience is making me a manual transmission fan even more. My dad is a mechanic for an airline so i'm pretty good at fixing cars but i've never rebuilt an automatic before, looking forward to learning on this 4L80E but any info on which 4l80 years are compatible, reverse diagnostics, i saw one youtube about a bushing burning a shaft down so that oil pressure leaked and wouldn't apply servo, really wish i could fix reverse with the trans in the truck but it looks like i'm tearing it apart soon. Looking to upgrade whilst it's apart, hd reprogramming kit, stronger clutches, not the expensive hardened input shafts but enough of a build to hold 6-700 hp would be sick since these LS engines hold a turbo so well. thanks so much for any help, i lost my license delivering pizzas and have been on a bus for like 8 years, i can finally get my license back if i pay sr22 insurance, if i get this truck going i'll do that and have a heater this winter instead of icy bicycle seats haha.