Big thunk in rear end

[worknprogress]

When I go from drive to reverse there is a pretty big thunk in the rear. Anyone know what the fix is ? differential or trany? I have 00 classic and believe its the stock 231 transfer case its automatic trans. I plan on changing the fluids soon if I can fix that annoyance at same time

[prerunner1982]

Have you checked the driveshaft Ujoints? That would be my first guess.

[4.3LXJ]

There are a couple of things to check. Motor mounts and tranny mount and how much play is in the rear end. As they wear they get rotational play and a change in direction can cause a clunk

[worknprogress]

There are a couple of things to check. Motor mounts and tranny mount and how much play is in the rear end. As they wear they get rotational play and a change in direction can cause a clunk
Ok how do I check the play. While driving it seems to shift or dip in a way. usually in turning from stop etc. Also when shifting if I completely stop & wait in neutral for a sec it doesn't do it as hard or at all at times.

[worknprogress]

Have you checked the driveshaft Ujoints? That would be my first guess.

negative honestly I didn't know where to start till the couple replies came. I'm going to try and greasing the slip yoke ill check um.

[4.3LXJ]

Ok how do I check the play. While driving it seems to shift or dip in a way. usually in turning from stop etc. Also when shifting if I completely stop & wait in neutral for a sec it doesn't do it as hard or at all at times.

Engine off, tranny in neutral. Twist the drive line while you are checking U joints and slip yoke too