does anybody know the reason why my truck does this?

Thats easy to test - if it is the tires, swap sides with them - put LF on the right front and the LF on the right. Same for the back. Now it should pull left. Simple to test, should take little time.

I don't think its tires, as badly out of balance as some of what I've run have been, I've never had one act this way. More possible is BrianG's suggestion that toe is off. Unless trx has changed, they come with 0deg toe on front and 1deg toe in on the rear. Caster is set with caster blocks or spacers, are they still there and in the same place on both sides? Camber is the angle the tire makes with the vertical viewed dead on. These would have to be pretty far out of whack to cause your prob, so it should be readily apparent.

Drive pins - there is enough tension on wheel axles to drive a wheel until you reach enough speed that a missing pin will prevent that wheel from staying in synch with the others - then it will veer off straight line. Pins are all or nothing. If one starts to back out while running, it usually hits bulkhead and you'll def know it. I have seem then fly out and not touch anything, pin's just all of a sudden gone. Again, an easy thing to check and eliminate as a possibility. Have you checked all wheel bearings? Another easy thing to check that could be causing this.