87chev4x4

Hey guys, I have got a 1994 chevy truck with a 60e in it. I had the trans rebuilt in march with a hd2 shift kit and a vette servo. Nothing really fancy as this is a low hp tbi truck. It has been doing fine then yesterday i noticed that when driving down the highway it was only in third gear. I thought it was odd. I got into town and checked fluid it is not burnt and is right where it should be. Now i have drove it today and it is fine when cold but when it gets hot it only uses 2nd and 3rd gear it will go into low if i put it there manually and the shift is crisp, but will not go into OD any suggestions? thanks