I have a 99 XL 5 speed 3.0 4x4 with the 7.5 rear and 3.73's. The factory size tire is a 215/75R15 and it has 235/75R15's on it now , i'm thinking about going with 31x10.5R15's. Is the 7.5 rear strong enough or should I stick with the 235's ?

 

I put 31" tires on my 4WD 89 STX almost immediately when I brought it home from the show room floor. It had almost 200K miles on it when I sold it. It did considerable but off-road duty, towed my race car all over the country and was my daily driver. I did switch from 3.73 gears to 4.10 after 50K miles because the taller tires with the 3.73 gears were too hard on my clutch. I also had to replace the wheel bearings and axles around 150K miles but I feel that was due to the heavy load on the bed (~800 lbs) of race spares, tools and gear for the race car when towing long disances. It had nothing to do with the 31" tires. The 7.5 axle held up just fine to the 31" tires but I highly recommend getting lower gear if you have a manual transmission. You should be able to get by with stock gears if you have an automatic.