I was wondering if I ran 15" tires and rims on the back and 14s on the front would this cause any  damage to the driveshaft and u joints? make them wear any quicker or break prematurely? or is it perfectly fine?

Thank you

lil_hustler_620,  On a 2 wheel drive  vehicle , it shouldn't matter if the wheel/tire diameter is different. On a 4 wheel drive vehicle, different wheel/tire diameters equate into different front and rear axle speeds, that would stress all the drive train members.  Depending on your driving habits, bigger/wider tires on the rear axle, giving you better traction, might suffer if you're agressive at accelerating. Again your drive train would suffer. John

Thanks for your input. I'm definately not building this truck for drag racing or to use it as a burnout machine. I like to conserve fuel and repair cost and re-use some old toyota rims and tires thus, running them on the back I don't have enough useable 14 tires haha . Mine is a 2 wheel drive model so I guess i could run 15s in the rear as long as I drive correctly.

Thanks