Charles is suggesting that you make the legs adjustable and use regular
casters. I rather like his idea, it would give you height adjustments up to
double the length of your leg.
So you would drill ( or route/dado a groove) a long hole in the bottom of
your leg to hold your threaded rod. Then attach a nut to the bottom of your
leg to hold the rod. The other end of the rod would be fixed to a small
block of wood and the caster would be screwed to the other side of the block
of wood. ... Perhaps you could even find a long threaded nut with a plate
attached and just screw it to the bottom of the leg.

Thanks for the explanation. Now, I can see that this is a good design.
I can use my hand to turn the wood block at the bottom to adjust the
table height without using a wrench. This is indeed a very nice
feature.

I might use two nuts a few inches apart in the leg to give me better
stability.

I don't understand this part.

Let's me explain what my understanding based on your description is:
- We drill a long hole at the bottom of a table leg.
The hole goes up inside the leg.
- We chisel a square hole on the bottom of the leg surrounding
the hole that we have drilled. The shape of the second hole
matches the shape of a square nut.
- Attach the square nut onto the square hole that we have chiseled
out using epoxy. We will screw a long threaded rod through the
square nut into the long hole soon.
- Prepare a wood block, and drill a long hole at the center of the
wood block and through the entire length of the wood block.
- Put a long threaded round-head bolt through the hole in the wood
block. The square area under the round head of the bolt will bite
into the wood and the long threaded bolt will not rotate.
- Attach a caster onto it using screws. The caster will cover the
head of the bolt. This is OK.
- Put wax on the exposed thread of the long bolt.
- Screw the long threaded bolt through the square nut into the
long hole of the table leg.
- Repeat this for the other 3 table leg.

I don't understand how the "two nuts" that you suggested fits into the
picture. I must have misunderstood something very fundamental.

Look forward to learn more from you. Thanks.

Jay Chan