01 3.4 making ticking sound in gear please help

My 01 double cab is making a ticking type sound. It seems to only do it under load when truck is put into gear. Makes the sound more when put in reverse and the truck has like a slight shake to it and kinda acts like its struggling. Only does the shake in reverse. Transmission runs and shifts smooth. Motor mounts are good. I don't have any clue what the shake is from and a buddy said the ticking sound maybe the timing belt. Anyone shed any light on this? It is more annoying than anything. truck has 126,000 miles and guy I bought it from said timing belt had been changed but you know how that is people say stuff