redtailhawker

My transmission started slipping in 4th gear a couple moths ago so my mechanic told me that as long as I wasnt going 70 or 80, driving in third would be ok and keep the problem from getting worse. Before I started having the problem I power tuned my truck as when I bought it, they gave me the one for the truck. I am wondering two things. First, could the previous owner have changed the gearing in the tranmission or rear end but didnt inform me so when I retuned it be causing the problem and if so what could I do to find this out. Second, I have the money to get the tranmission redone and wanted to know if there was anything that I could have the mechanic do to increase the performance and durability of the transmission that wont cost me an arm and a leg. The truck has a 5.0 vortec, 116,000 miles. Any help is appreciated, thanks in advance