Jay About 15 years ago, I
bought well used Spicer 5831 and installed it in my 1971 Dodge D300 ( ! ton dually flatbed). These trans were developed and sold by Brown-Lipe
and I think Spicer bought them out somewhere along the line. Either way, they are old. I took mine out of the Dodge last year and installed a Gear Vendor Overdrive as the Spicer was getting very noisy and had a bad vibration. FYI Spicer made 8 variants of this model-- all look alike from the outside --18 13/32 long and weigh 155 lbs. Starting with the 5831, which you apparently have, they made the 5831A through 5831G. The difference was in the three possible ratios. and NOT ALL of the combinations had OD. The straight 5831 is listed as 0.73, 1.00, and 2.00 to one. Parts can be a problem, I fear, although I never had to look for any. Good luck with it.
Life  is like a roll of toilet paper the closer  you get to the end, the faster it goes!  Enjoy each day.