Can you replace your 235 75 R15 XL 108S tires with 235 75 R15 XL 109T tires?

Yes, the only difference in these tires is the load rating and speed rating. The S is rated to 112 mph and the T is rated to 117 mph. The 108 is rated to carry 2.205 pounds and the 109 is rated to carry 2,271 pounds.