Why do i lose RPMS in overdrive?

I have a 1993 Chevrolet K1500 350 with a 5 speed manual transmission. When ever i shift from 4th to overdrive i lose all my rpms and truck starst jerking forward. I shift when im between 3000 and 4000 rpms and it still loses power. Any ideas why its doint this. I was told i have 3:10 gears in my transmission because i can go 55 in third and only be at about 2500 rpms. I have no catalytic converter on my truck either.