no you dont have to replace the whole rear...you can just replace the posi unit with a new one (that is rebuildable with available parts). Summit has them. I was lookin at the a couple nights ago. They are about $450
mine does it at 45....if i crusie at 40 stays in 3rd...if I cruis at 50, stays in OD.....i miss crusin at 45....lol....it does it no matter where I adjust my TV cable.