having problems with my 88 stepside

i bought the truck a couple of weeks ago. the tranny was wworking fine when i got it but after driving it about 60 miles i lost 3&4 gears. i replaced the tranny with another 700r4 and still doing the same thing. i was driving the truck today and engine just died and when i got it restarted it had a horrible knocking noise. hopefully someone can tell me whats wrong with it.