Dug our the mud hole in my back yard this evening and found a cracked T coupling. What is the easiest way to repair or replace a T in a sprinkler pipe?

Will JB weld work?

Buy 3 couplers and one T coupler, enough pipe and pipe glue to repair. Cut about a foot on each side of the T coupler and use the new couplers and pipe to piece it back together.

for a minute if you let it cure...

but seriously, its a simple cheap 5 min job involving couplings, glue and nipples. youtube will show you

Telescoping coupling and use blue glue.

slip coupling you mean? primer first, do it well.

NO!!!
A slip coupling is asking for a failure.
A repair coupling is a different animal. It is two pieces that sleeve together with one, and often more O-ring seals. It has a telescoping feature. You collapse it to minimum size, cut out that length of pipe less the sleeve joint at one end, and glue it on one end of the pipe being repaired. THEN you prepare the other joint to glue, and expand the repair coupling to seat the fitting at the other end. I have used repair couplings up to 3" in diameter on systems with 90 psi, and they do not leak when properly installed.
3/4 inch repair coupling at home depot

dang, I didn't go back to OP post, I saw only the question asking if a slip coupling was the same as a telescoping coupling. The T makes it different.

Nipples? Did you say Nipples....

When you dig down, make sure you do it very carefully as your zone wires might be running through there.

It may be cheap but I don't think in 13 yrs I've ever fixed a sprinkler leak in 5 minutes.

my record is 6 mins. but I just ripped out the sprinkler head and let er go full flow. worked for about a day.

How will this fix a cracked T coupling ?

1 on the end of each nipple.