Welcome to Justanswer ! my name is XXXXX XXXXX i will do my best to assist you with this problem
so all the forward gears worked just fine just reverse acted like it was in neutral when it was physically shifted into reverse ?
Gary : The truck runs well going forward !!!
The engine races up as though you were in neutral
or park ?????? while trying to back up ?????
ok please don't shoot the messenger here but these 4L60 use no electronics for reverse to work
even if all the electronics are disconnected off this transmission it should still have reverse and a forward gear
internally with no reverse either the low/reverse band has failed, the reaction carrier has failed or an outside chance of the planetary gear failed
im sorry to say its going to have to come out and apart and rebuilt or the cheapest way out is to install a used/salvage yard transmission as most come with great warranties