On the farm, if you were going to manual swap it, I'd go for a 4 speed. They are reasonably cheap because people want over drive. They are tough and with that granny low, if you can't move it, you probably shouldn't..
Ain't no creeper gear behind a diesel unless you put it there yourself. Meaning most any 4-speed with a diesel bell will be a close-ratio T19, unless the owner combined said diesel bell with a wider-ratio gasser trans. Close ratio has 4:1 first gear, if you gonna pull heavy often with lots of stop and go you absolutely don't want that with 3.55 gears, maybe 4.10s will make it more manageable but overall it still sucks especially in 2wd version (no t-case for the low range).
Best bet would be a diesel bell (requires flywheel and clutch also), a gasser 4-speed (for the low first gear), a BW1356 t-case (for the slip yoke), and a '93-up rear axle (for the stronger pinion). For gears 3.55s work well with a NP435 or T18 transmission cause of their creeper 1st and reverse, truck also still remains very road-friendly up until 55 or so. Creeper gear with 4.10s in the axle will pull just about anything, or break the U-joints while trying
Wouldn't waste any money on a ZF5, in stock form that thing is horrible for a yard/farm truck - it's like a close-ratio 4-speed but now it has a useless reverse gear on top of the crappy first, so you'll be in low range in the t-case all the time, and if you don't even have a t-case well kinda sucks to be you. Some folks modify big-block ZFs to correct for the lousy diesel gearing, or you can swap the big-block gears into a diesel case, either way it's not cheap and really it's not worth it unless you actually have a need for OD top gear.
The lunchbox (aka lincoln?) Would do fine.
Lunchbox is the type where you pull the gears from inside your open diff and install the aftermarket locker in their place. Lincoln is an open diff that has been welded, for best results to both gear to gear and gear to case, last part can be tricky tho cause the case is cast so it doesn't MIG very well, stick is better.
I would go for the 4.10 or 4.56, there good for pulling. welding the spider gears is a cheap way to get a locked rear end. I would keep the c6, there cheap to rebuild.
Factory 4.56 axle are rare, his best bet for cheap axle will be a 4.10 axle. Which is more than fine if he drops in a creeper gear 4-speed or 4x4 or both. Agreed on welding the open diff, the factory LSD is not very grabby in these axles, and on soft terrain a welded diff won't wear the tires like it kinda does on pavement.
It doesn't help that the C6 is so weak that you have to hit 1200 RPM before the truck starts moving. I can pick up a axle from a 96 F250 (5.8L) with 3:55 gears for $150. I am about to retire this truck from DD duty to be our farm only truck so no more highway running, but more towing. I like the easy service brake drums on the 95 axle but wonder wha you guys think. Any year better that others, what about ratio, and is the factory limited slip worth looking for? The truck has 285r 70 x 17 tires right now.
