Hello,
Going down the interstate we began hearing a banging sort of noise in the rear passenger side (around the tire) of our 83 300td. The banging would get progressively louder and thunkier, we would slow down to stop and the clunking would slow down but get louder then stop. we would start to drive again and the noise wouldn't come back for about 20 miles then we would just stop and go again for another 20 miles over and over again. Any ideas what this might be? Any suggestions would be helpful.
Thanks
stephanie

Bad axle.
I can almost guarantee it.
Feel around the boots on either end of the axle... you will find a split in one of the boots, from which all your grease escaped and dirt/water got in.
You will need to replace.
That said, they can go a while once torn before catastrophic failure.
A bad axle can sometimes be "reset" by pulling over, reversing for 20' or so, then going on down the road. Will start clicking/thunking again in 20min or so.

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How much would that cost?

If you have do not driven much since noticing the problem just replacing the lubricant might stop it. After too many dry heated miles it will be totalled. Same if it is filled with road dirt. Much depends if the boot has cracked all the way around and is open. Or just a crack that let the lubricant escape but the boot is basically still semi sealed.