Unfortunatly the only way to uncover the problem is to examine the drive train. Start with the u-joints, looking for looseness by rotating back & forth. This is not always easy to spot, there should be no rotational play between the two parts.
Next check the trany mounts, these are often overlooked & are often worn or broken (soaked in gear oil).
Axle spline wear is pretty much a given on most cars but you usually won't get it in the higher gears. You should also remove them for inspection & repack them with a quality moly base grease. This helps with hard shift binding as well.
The diff mounts are a weak point & need to be looked over carefully. Check for cracks at the "bridge" where the stud passes through. If these are in need of repair TRF has a pdf template available on line to make steel plates to box these in & reinfoce the mounts.
Last but not least is the diff itself. This unit has a bad rap & is certainly not the beefiest unit ever made but I know of many hard driven cars with over 100,000 miles on original units. It's a crap shoot to get a good one it seems. The side carrier spider gears have a fiber washer that wears .
There's alot to look over & the drive train has always had slop issues, so unless it's unusally bad it just might be normal, what ever that is.