If you have 31" now, I assume you probably have a 4x4. If so, I would stick to the 31" as smaller tires will mean higher RPM (more gas) as you do mostly highway driving.

If you want to save a few bucks, try to find a tires that starts with "P" with the same overall diameter of 31" that is more suited for passenger cars. The benefit of such is that the tires most likely is lighter, and narrower than the standard 31x10.5/15. The narrower foot print would mean less rolling resistance. The drawback is that you won't be able to carry the heavy loads your truck is built for, and the sidewall maybe weaker if you take it offroad.

Another cheapie idea is re-treads. The re-tread industry has improved quite a bit over the years. Most trucks and commercial fleet vehicles are all running re-tread tires. They offer similar performance, but will save you about 20% or so in cost. Not a lot, but some savings.

Lastly, you can always look under local classifieds for used tires that are factory take-offs. Many people that buys new trucks may want to upgrade their wheels and tires immediately. Sometimes, you can get a very good price on an almost new set of tires.