Suggestions for Upgrades

I have a 2000 Dodge Ram 3500 that is bone stock, automatic, 4x4. I have owned the truck for a year, and have made no modifications to it. I mainly use the truck to pull my horse trailers. They are 38 ft long and can hold either four or five horses. The truck does alright towing. I seem, however to loose power on hills and so forth. The main purpose for the truck is towing, however i wont to modify it a little. I have no codes or anything at this time on the truck, that may indicate a proplem. I want to truck to not hesitate on the hills, which it shouldn't. I understand it is automatic and that may give me issues.

Where would you guys start with the modifications and to reduce the power lag. Exhaust, air intake, filter, turbo, etc?

Thanks,
Corey