I have the following axles:

one Dana 60 front (4.10 ratio)
one Dana 60 rear (4.10 ratio) (old...16 sp shafts / small housing)
two Dana 61 rears (3.73 ratio) (BIG housings)

I would like to run a 61 housing... rather than buy another 60.
Everyone says I can't swap the 4.10 gears into the 61.

Everyone = Dynatrac, Randy's R&P, my local bolt on shop, and
people I wheel with.

nobody seems to know what is diferent, they just seem to agree that there is a diference.

Do any of you guys know if / what the diference is?

(it seems to me I should just be able to Linde-lock the open 61 carrier and run a ring gear spacer on it with the 60 gears.)

'83 Toy 2wd long box Ford 460, C-6, 205, Front Dana 60, Rear Dana ??

4am so cut me a little slack if I'm not 100% correct :D

The difference between the 60 and the 61 is the pinion to carrier orientation. The Dana 61 was designed to run gears in the low 3's all the way down to something like 2.75:1. Had to meet those new fuel mileage requirements of the 70's ya know.

This resulted in a much larger pinion gear and the ring gear would have been way to thin. So they redesigned the pinion to carrier orientation and used a carrier with a greater ring gear off set. You would never be able to drop a set of 4.10's in because the pinion would never come in contact with ring gear.

Might have smaller brakes than a regular D60.

Or I could be total wacked and need more coffee....

Not 100% sure either, but this is what I remember seeing.
You can buy a spacer for the ring gear to run D60 gears. But, I don't know where the cutoff is for ratios

I ran into the same problem. I cut off the the big spindles on the 61 and put them on a 60. A good shop can do it for around $100 if you cut everything first. Then you can slide 35 & bigger spline axles right in. The disc brake setup will be the same as on a 60. I found out spacers existed but according to some shop's they were weaker because of the longer bolds used or some kind of BS. It's just seemed easier to me to put on the bigger spindles and standard 60 components. It was a good decision because even with spacers all you can go to is 4.56's in the 61 housing and I'm dropping to 5:13's.