the pikup over heated and the engine light came on now the pickup will ot shift.
Shifting problem on 1990 GMC 1500 Pickup
your transmission filter is plug up. it will cause the engine to heat up, as the tranny is running hot and slipping,hope the disc are'nt burn. pull the tranny dip stick,if it smells really bad,it might be too late.do not have it back flush,it screwed my tranny up. you need to remove the pan and clean the bottom and inside,replace the filter ,refill with new atf,run engine and top it off. good luck