93' 3500 4x4

Mainshaft output splines and t-case input splines stripped out a while back. So....I replaced the planetary in the t case and the main shaft in the NV4500. Which fixed the grinding and skipping problem from the stripped out splines.

However, I took the truck for the first test drive and ran and shifted fine up and down the driveway, but within 300' of taking off from my driveway on the main road, I felt the truck bogg down like it was under a big load, and boom! Rear tires locked up. Truck wouldn't budge in gear, until I found out it was permanently stuck in reverse, so when the shifter is in neutral, it is still in reverse. I can drive it around fine in reverse, but no forward gears, as it puts it in two gears at once.

Popped the shifter and made sure shift tower was not engaged into any gears.
Popped the top cover and reverse slider looks like it is in neutral (all the way to the output shaft side), but the shaft is locked up. It looks like the reverse synchro/teeth got shaved down and that fresh expensive oil is now black...also looks like shaved metal in the case from the reverse synchro, but all other teeth/gears are fine.

I will pull it apart in the next week here, but wondering if anyone has any insight on what might have happened? I have the shop manual and made sure everything went back on the way it came off, set backlash to 0.002" on both shafts, etc...