2.56's are good for one thing, and one thing only. Creating a smoke show. Once you get a tire going with those gears, it is extremely hard to get it to stop. Well, there are a few other quality's. Like towing on the highway in 3rd gear. RPM's are still not that high, and saves the tranny. Or if your 4L60E becomes a powerglide, you don't have to get it fixed right away. I put 3,000 miles on my 95 Red Roadmaster beater with no 3rd or 4th gear. I still drove it on the highway at 65mph. At the dragstrip, I was crossing the 1/4 mile trap in 2nd gear!
Really, 2.93's or 3.08's still offer very good gas mileage, with an improvement in off the line performance. 2.56's used a series II carrier, than only supported the 2.56, 2.41, 2.14 and "higher" gear ratios. 2.56 was the only from that used in the 91-96 Roadmaster. The series III carriers covered everything else such as 2.73, 2.93, 3.08, 3.23, 3.42, 3.73, 4.10, 4.56, 4.88, and even "lower". You can use a series II carrier with series III gear sets, but the ring gear requires a spacer. Series III carriers don't work with series II gear sets. The pinion head is way to big.
The only car to have a positraction with 2.56 gears, was the 94-96 Roadmaster SEDAN. It was optional, and on about 1/4 of the 94-96 Roadmaster sedans. I have only come across a handful of them, while looking at Roadmasters FS or in the junkyards. Not sure where they all ended up. You can swap the carrier if you find a good used one. Just need to re shim it side to side to get the pattern / backlash right for the gears. Or, and you are more likely to find a posi still working with an entire axle swap with one from a 91-96 Caprice sedan/ Roadmaster sedan.
The sedans used a housing in two widths. The 93-96 Civi Caprice / Impala SS used the wide housing. The Roadmaster sedans, 91-92 Caprice sedans, or 91-96 Caprice 9C1's used the skinnier housing. The wider housing will work, but it will push the tires closer to the fender, which will make it harder to get the tires out, and also will take away from the width of of tires you can use on the stock rims, or rims with a similar backspacing. The stock positraction was not known as being extremely long lasting. Some are junk by 100k and acting like an open diff, while others are still strong at 200k. I have seen them every where in between. I have had ones that were garbage around 100k, and others that would still light both rear tires up at 220k. The one in my 95 RMS "Jesus", has 86k on it, and is weak sometimes. One of the Caprices I had with 150k, that posi was still extremely strong. they can be hit or miss. Aftermarket ones can be had for a series III gear set, and run about $300-$500 depending on what you go with. I have a clutch based Eaton in 95 RMS "Buffmans car", and my 96 Caprice 9C1. If I did it again, I think I would go with a detriot locker, and never have to worry about replacing it again. But, they are expensive.
2.93's/3.08's are still really great on gas, and much easier to find with a positraction that is still working. My 94 Fleetwood had 2.93's(same engine/tranny) and got 24mpg from FL to MI. knowing my dad, he drove 65mph-75mph depending on the speed limit. 2.93 posi was the setup used on the 94-96 Roadmaster/Caprice sedan with the towing package. 3.08 (posi optional) was used on the 94-96 LT1 Caprice 9C1. The L99 Caprice 9C1's were 3.23(posi optional). Some 91-93 Caprice 9C1's were 3.42(posi optional). 92/93 Roadmaster sedans with the towing package were 3.08(posi optional). There were other 91-93 Caprices with 3.08's. But, the civi caprice sedans were the wider housing that is worth noting. The Impala SS was 3.08 posi on the wide housing. All of the 94-96 Impala SS/ 9C1's had a rear disc setup. In stock form, it was crap. But a cheap and easy mod makes them more effective.
The Caprice sedan also used a 7.5" axle behind the L99(4.3) / LO3(5.0/305) caprice sedans that were not 9C1's. None of them are positraction, but you don't want one of them even if it is free. Well, unless you use it for a paper weight.
I pulled a 93 Caprice civi axle, that is a wide housing 3.08 with a surprisingly strong posi. It's going under my winter beater 95 Red RMS, so tire width is not an issue. Plus, I want to get away for the horrendous 2.56's on top of adding a posi for snow. I also pulled a 93 9C1 axle that is 3.42 posi for my 95 RMS "Jesus". The 94-96 Fleetwood had 3.42's in the towing package, and they were still rated at 22-24mpg(forget exactly) highway, IIRC. I have also pulled a few 3.08 posi axles from 94-96 Caprice 9C1's. I go get mine on a half price day at the U-pull it yards. But even at full price, they are cheap.
I can normally pull one in a yard, by myself, in around 2 hours. The hardest part is cutting the parking brake, and then attempting to lift it in the wheel barrel. I'm not very big, and lifting HALF of a loaded 8.5" axle is hard for me. There was a write up on auto blogs, on doing an axle swap on a 94-96 Roadmaster sedan.
Don't bother with wagon axles. They are even wider, and have hte LCA mounts spread farther apart. But a carrier from one will work.
~Cory Magner "My name is Cory, and I'm an addict."
95 DCM Buick RMS 14.21 @ 91.79 "Jesus" Blown Engine at 85k
95 DCM Buick RMS 13.52 @ 99.33 "The Doctor" Blown Engine at 180k
96 DGGM Buick RMW. 263k, still on the factory tranny.
And 5 other LT1 powered land yachts Is there Rehab for this Sh*t?