hear a squeek when reversing

[red72chevy]

here's my problem,

when my truck is engaged in reverse and goin backward i hear a squeek. its horrible, and it isnt constant, i sounds like its rotating cause it goes faster then slower. is it my U-joint or what???? it also does it when i downshift from drive to 2nd (auto tranny). whats the deal? any help would be appreciated cause im a dumb 19 year old.

1972 chevy p/u longbed-grandpa's first new truck
350-72 impala engine donor (6oK miles, honned, new rings, new cam, like new)
edelbrock 600cfm carb
high rise polished intake
flowmaster exhaust
350 turbo tranny
cragar s/s rims
viper red