Check for leaks, even being a little low can cause 3rd to start slipping, uses the same clutches as reverse, but reverse uses a higher line pressure than in third. After 71 they changed the seals to teflon, which become brittle after a while because of heat cycling. Just make sure it is to the top of your full mark, not in the middle, sometimes with older vehicles, especially diesels can wear out the transmission mount causing the tail of the trans to sit a little lower than it is supposed to, causing the fluid to all move to the back of the pan. While the dipstick and filter/pickup, is toward the front of the pan. So anyway, just make sure it is full, it may not being getting enough fluid if it is not.