there are MANY different issues that add to this including the tread type. if you can, search out the options that were available from the factory, and look for the largest size tire they offered, rule of thumb is you can safely go one size TALLER than that. Width however is determined by your wheel offset AND tire height, and that doesn't have any "rule of thumb" and is where you end up having to deal with tread design, for example a mud tire will throw more issues than an all terrain due to fender clearance and the tire lugs height up the sidewall. This is where your best bet is a good tire shop, and the actual measurements not the tire sizes, of the tires. If you know someone who "did" this or that, it doesn't mean it will work for you, due to modifications over the years to the tires used (here is where the actual measurements are needed), suspension issues (age fatigue so on) sagging axle housing and so forth. So if you want to go taller than 31, ( you are there all ready) you need to address the suspension first off so you aren't bottoming out when loaded, and make sure everything is up to spec, based on your comments of current issues. At that point you may want to look at a simple 1 3/4 -2" max lift to go up to 32 inch tires. You may need to change your fender flares and possibly your wheels for a different offset at that point.