Dana 61 Tech- Can it be Geared, what are the advantages
Tech:
Most of the rears are the big spindles and can run 35 spline shafts without being bored, not all check them to be sure before getting to involved! Most of the 61's we have seen come with 3.07 gears, in order to run the 3.07 gears and fit the monster pinion head in there dana changed the offset of the pinion by changing the housing not the carrier. Dana 61's did however come w/ 3.55, 3.73 and 4.10 gears. The best advatage to these, they are cheap b/c the Q of whether or not they can be geared is a bit mystical. And the good news is they can be regeared!
Gearing:
This is how you get one to work. There was a a dana 60 spacer made by Reider that allowed you to run the lower ratio gear ie 4.56 and up. I believe this spacer was 5/8 thick. It required the 4.56 and up dana 60 carrier and the ring gear spacer to get the offset needed. This said spacer seems to be just a legend now and has been discontinued. However their still is the Mr. gasket dana 60 spacer (5/16 thickness) that just about every drivetrain company sells. This spacer is designed to allow you to run the 4.56 and up gears on a 4.10 and down carrier for a regular 60. This spacer does come into play but need not be confused w/ the legendary Rieder spacer, they are of different thickness.
Alas there is a dana/spicer Dana 61- 3.33 and up carrier part# 706400x. This carrier is 2 3/4 tall. Where as the dana 60 4.56 and up carrier part# 706041x is 2 7/16 tall. So what combination do you need to run which gear ratio? If you have a dana 61 that has 3.07 gears in it and you want to run 3.33-4.30 gears, you will need to run the dana 61 3.33 and up carrier part# 706400x or the dana 60 4.56 and up carrier with a 5/16 ring gear spacer. This is popular for people who are trying to regear a 3.07 dodge cummins and the like front 61 that has a non overdrive auto, If you switch to a manual or to an oerdrive auto transmission, you will likely want some lower gearing that is how you get it.
If you have a dana 61 with 3.33 or higher gear ratio all you need is a 5/16 ring gear spacer to run 4.56's and up.
If you have a dana 61 with 3.07 gears and you want to go to 4.56 or numerically higher gear ratio you will need to purchase a dana 61 3.33 and up carrier part# 706400x and a 5/16 ring gear spacer to achieve the desired offset. Or have a 5/8 ring gear spacer custom machined. This is popular for the guys trying to score a cheap front or rear 60's and gear it for offroad use.

Thick Gears, Carrier changes
No thick cut gears are available for the Dana 60 regular rotation axle ie rear dana 60 axle.
There are however thick cut and thin cut gears available for the high pinion reverse rotaion 60's. In order to run 4.56 gears or numerically higher a ring gear spacer or a carrier change to a 4.56 and up carrier is required for a regular rotation 60.

Cool, thanks for posting it!  I'm glad I gave credit where it was due so I didn't look like an ass, lol.