I have a 1981 chevy cheyenne, and am looking for new tires. The tires I have now are 235 75 R15. The thing is, we don't have rural running water, so I haul our house water from a pump in the local town about 8 miles away. I have a 350 gallon tank that I haul water with about twice a week. I need to know what kind of tires can hold up to the weight of all that water fairly easily, and on 15-inch rims.

Can someone help me?

You should take a look at tirerack.com, I did a search for your size of tire and this is what I came up with...

Tire Search Results

D rated tires which should work... the other option is to get a cheap set of 16" rims... that would make your life a lot easier because then you could upgrade to E rated tires... basically more load capacity. Hope this helps.