I have a 1981 F-150 Ranger XLT with a 351M and C6 transmission.When it is cold it will no go into drive until the truck has ran a while and warmed up. Now that winter is here I have to let it run 15 to 20 minutes before it will go in . It drops into reverse with no hesitation.

Hard and shrunken forward clutch piston seals.

That happens with age, mileage and heat.

One reason reverse is stronger is the increased pressure to apply reverse vs forward.

Thanks for the info . Looks like a tranny in the future .