I talked to Denny's Driveshafts myself monday and I had decided to go that route myself, they told me the same thing that when you run low gears and high highway speeds that a standard shaft can't handle the strain, my T56 has a finer spline and smaller dia. then my 5 speed trans. so I was going to have to buy a new yoke and get my other shaft modified but with the problems you have had and the money it would take to modify mine I decided to go with Denny's. I printed the forms they have on there web page to get the proper measurements but it will probably be Sat. before I get the engine and trans. stuffed back in to get measurements. Let us know how your shaft works out, they told me 2-3 weeks turn around time.