help please....TH 400...1990 1/2 ton

My 90 1/2 ton has a TH 400.My cruize controll has never worked.I read somewhere that If a torque convertor is not working properly then the cruize controll will also not work.
when i am slowing down to come to a stop the transmission seems to upshift when im at a about 5mph.other than that all other shifting seems to be ok.It seems to shift just fine when im picking up speed.I am going to check the Modulator valve.So could this be the reason the cruize controll does not work and also what else can i check??

umm... did that truck come with the turbo 400 or did you replace a 700r4 with the 400

they had the 400 turbo option til 91. that's when they got rid of the r/v style and the 400.