[tirebob]

Different vehicles and different driveline systems all react differently to mismatched overall diameters/circumference of tires....

Generally speaking, on vehicles that operate primarily in 2wd (most 4x4 trucks are 2wd unless locked in 4x4), the traction wheels are you want them matched to a close degree. If it is not, it will not cause your vehicle to catastrophically fail or anything, but it will cause premature wear of the related components over a longer period of time. Not a big deal in the shorter term, but long term it could cause you to spend more money in other areas sooner than maybe you would have. Mismatching on the front of a rwd is best for the wearable components, but then you may be fighting a constant pull on the steering. Not really damaging but potentially annoying.

Now on a full time AWD it is not ideal for sure to have mismatched diameters because those system redistribute power around to maximize traction in corners or if the vehicle detects slipping etc, so with the improper diameters it will be potentially tricked into misdistributing power around because of the different speeds at which those tires will be turning which in turn will mess with the optimised handling and also prematurely wear related components. Again, not a huge issue in the sort term, but possibly expensive wear long term. On awd vehicles, you want to keep the tires on the same axles very, very close but front to back can be a bit more variance for sure.

Again, some vehicles tolerances are very tight while some can handle a fair bit more variance. My point is there is not hard and fast rule. On the OP's truck with 50% wear, I would absolutely recommend a pair for sure, but if it is not locked in 4x4 for any real long and regular use I would say he doesnt need four, but if he does use 4wd a lot on pavement I would be doing four if I owned it. Locked in 4wd on dirt etc though it isn't near the issue.