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I have a Murray 1989 mod 9-38608x92 lawn tractor 12hp/38" cut.
Recently started throwing the drive belt off the rear pulley. This
Happens when stopping, putting in reverse, back to forward-then off it
comes. Is there an adjustment for this or is something likely worn out?

Most likely the front transmission support bracket bolt has came out. Most likely there is a bracket hanging down on the left side of the tranny where a bolt attaches it to the tranny to prevent twisting when starting and stopping. I have also seen this bracket on the other side, and on both sides.

How old is the belt?
If the belt is worn out it will do this.

Also, push the pedal down and engage the parking brake. Then get off, and get down, and reach up to the trans pulley, and wobble it up and down
and check the slop, the input shaft bearings get worn out on these after while, and they make a kit.

As well, look at that 5th bolt point like ILENGINE pointed out, as that is a good possibility. Also, there for a while, Murray had a big problem with the
metal of the tractor body cracking badly at the area where the transmission bolted on, and causing belt troubles.

I found a problem. The part of the transmission that the support
Bracket bolt screws in to is broken off. Should it be welded back on
Or would J B Weld work?

Also, the flywheel pulley has play & the idler pulley mechanism has play.
And I replaced the belt which didn't help.

JB weld won't hold there.

But what did you mean "flywheel pulley"? The pulley on the engine? If so, that is very bad...

If you are talking about the transmission pulley, that is to be expected.

I meant transmission pulley (don't know where flywheel pulley came from)

Can the piece be "welded" back onto the transmission? It's cast aluminum
As you know.

It could by a competent shop.