Interesting how personal bias (not facts) gets mentioned by a few. If I was going to use bias, not facts I'd say the 4500 that James is talking about is a PIG of a transmission- same with the ZF, but I won't. I will say they have their place but it's not when you want momentum and keep your tire speed up- fact, not bias.
If you never need momentum then any wide ratio trans (those with huge gear splits) will work pretty good.
Another personal bias "... But no one is out there breaking their Bronco's anymore. So yeah, turn your Bronco into a TJ and bounce curbs at the Kroger." Curious- Why even say this?
I have run 9 different transmission/transfer case/doubler options in the same Bronco over the last 6 decades ('70's - 2022). I've done the stock stuff, toploader 4 spd combo's, 3 spd OD's, ZF's with doublers & D20, ZF with doublers w/Atlas (loved this combo but not anymore), C4's, 4r70's with doublers with/without NP205 and 6r80 with Atlas. Steve convinced me years ago that extremely low gearing can be hazardous- and I've never looked back! (thanks!)
Doesn't matter what works for me- this is about YOU and your wheeling/driving style.
If you crawl in the rocks you need something different than a mall crawler would. If you play in the sand or deep snow you do not want a transmission with huge splits because you almost come to a stop even shifting as fast as you can (which isn't fast).
I drive almost everyday comforrtably (very important adverb) at 70mph and around our country roads and thru our little town. Since I only have one driveable Bronco and I use it for everything.
I think I have found (for me) the best trans and gearing combo for what I do. Deep snow, sand dunes (not packed beach sand), highway, DD (like 175mi/week daily), light trails and rock crawling like Sand Hollow/Moab. This is my opinion after decades of experimenting with 200+:1 crawl ratios and other gearing options. Not saying this is for your driving style.
Many benefits but I haven't experienced a drawback yet with the 6r80, 3:1 Atlas combo.- caveat is - the rest of the Bronco is built around it. Engine size, 40"s, 5.13"s all compliment each other without any drawbacks but this is for my style of wheeling/driving-not necessarily yours. With that in mind a big truck transmission that James likes might be perfect for your intended uses but holy cow- saying one trans with the smallest weakest transfer case coupled to it & available for a Bronco is the best is a bit out there... wow.
I'd never run a D20 again in anything but a street rig that only sees light at best, off road duty. They were built for 28" tires, 170 HP, 3.5:1 gears and a 3 spd. No torque there compared to what most of us run today. Torque kills parts.
Apologize for this being book length but the correct trans/t-case/engine/ tire size/ axle gear choice makes or breaks (literally) your rig and will ruin your vacation when the weak D20 breaks when on that once in a lifetrime trip to Moab or wherever. It happens often and I see it every year at Moab. Trips over when the D20 explodes.
Best of luck in you choices- it's taken me 46 years.