If i replace rear shocks do i have to replace feont at the same time?

You can just replace the rears then the front at a later date. Being that its an older truck, your front torsion bars might have worn out and caused the front to sag lower then when new and the fronts might be harder to put in. if thats the case, your ride might be different and it might need an alignment. Its not required though or recommended. If you mess with the torsion bar keys ( usually under the driver on the frame cross member) then yes an alignment will be required.

You do not have to replace the front shocks if you replace the rear. You also do not need an alignment when you replace the shocks on the front or rear. Shocks have nothing whatsoever to do with alignment.

You have to do both rears at the same time and both fronts at the same time, but it is not unusual to do just the front or just the back if the other pair is holding up. On a pickup it is not unusual to have one pair last twice as long as the other pair. My work truck sees a lot of off-road and not a lot of hauling, so we go through front shocks about 3-4 times as often as the rear shocks. Hauling on pavement reverses that.

no

You don't HAVE to, but If they're old It's a good idea to change them all at the same time or you will get an uneven ride. Don't be such a cheapskate. You should get the alignment checked periodically anyway, whether or not you change the front shocks. It's a good maintenance practice.

After changing shocks it will be best to have the wheels aligned. If you want to do that twice, stagger the job on the front and rear. Be smart, do the front and rear together and align the wheels once.