Busted Differential...

that's where mine broke too,
I got a rear diff from a 3 speed truck which was higher geared than mine,
my panel was made when line guys were hung over,
had lower 4 speed diff, 3 speed on column, but had a sticker with 4 speed shift pattern on dash,
left factory as a 3 speed, not later made to one,
quality control issues even back then.
anyway it was a lot peppier with that 3.73 instead of 4.11 axle