'80's Rangers have a 7.5, as the 8.8 wasn't introduced to RBV's until '90/'91. '90's Rangers got both 7.5's and 8.8's. I'm pretty sure all 4.0 Rangers were outfitted with the 8.8. I'm also pretty sure that most 4 cylinder Rangers would have had the 7.5. What I'm not sure of is if the smaller V6 Rangers (2.9/3.0) would have gotten a 7.5 or 8.8 or either depending on other options.
What are you hopiing to accomplish with the axle swap? Are you just trying to change gear ratios, and you have a separate lift kit so you can fit the 33's or what exactly are you after?
Remember, if you're changing gear ratio in the rear (and you have 4wd), you also have to regear the front, too.