A few things I'll add on my knowledge of the 9 inch.
The axle flange studs do not determine whether it's big bearing or small bearing. Big bearings came with both the 1/2" AND 3/8" T-bolt flanges. The smaller 3/8" were more common in the 80's years on truck applications. I don't know about earlier model year car applications.
Determing big bearing or small is best done with a caliper at the wheel bearing. Big bearings were very common in the F-150 applications.
As to running a 33 spline...
You can run a 33 spline with the OEM third member BUT your only option on carriers is a full spool.
Something to consider too; if you already have a 31 spline shaft, why spend the additional $350 for new shafts and wheel bearings and another $150 for a new spool??? 31 spline shafts in a Cherokee are still an extremely stout shaft size for nearly all kinds of wheeling.
I ran 31 splines with 38's in a Scrambler for years behind 100:1 gearing and a V8. 31 spline 9 inch axles are likely plenty strong for your application.
As to discs: the Eldo calipers are not cheap. Typically they will run around $150 ea rebuilt. There are cheaper options out there including running a front brake caliper, but this will mean no E-brake. Explorer rear discs are also a good one to consider. They have the e-brake and the rotors are 5 on 4.5". There are several caliper brackets available on the market but making your own is not too difficult.
Bolt pattern is something you need to consider. Ford trucks have the 5 on 5.5". You can have the axle flanges (and drums/rotors) re-drilled and press in new studs to convert it to a 5 on 4.5" pattern.
There is no reason to go full floating in my opinion.
Detroit or spool is a matter of preference. I've run both in 9's. No doubt the spool wears tires faster on the street.
Width also needs to be considered. Very, very few, if any, of the narrower car housings have the stronger "heavy duty" housing. Most all narrower applications had the "banjo" housing; including vintage Broncs. The heavy housing is much stronger but would be wider than your front axle. The banjo housing could also be reinforced and trussed to increase it's strength.
I would just add too that cheap 9 inches are plentiful out there. If you desire to stay cheap you need to find one that best fits your needs with the least amount of modification; gears, bolt pattern, width etc. When you start building up a 9 inch with new shafts, rear discs, etc; it is no longer a cheap axle and you would probably be saving money going with an Explorer 8.8 which already matches the XJ/YJ width and has discs.
