The transmission is fine, they ran basically the same trans behind the SOHC 4.0L that's almost double the horsepower... I'm running basically the same trans (different case being a 4 cylinder) with ~225hp on my turbo Ranger...
I'm pretty sure you have a 2 wheel drive by the sounds of what you are talking about, which makes things cheaper since you only have to deal with one differential. Bare minimum would be 4.10's which can be found in some Rangers which is cheaper than regearing. But if you have to regear I would probably just go to 4.56's, it'll feel like it has more power than stock unless it came with 4.10's...