Ok so I sent my xj to get fixed, but now when I get on the expressway and drive 65mph for around 20 min it starts to loose vacuum on the air vents then starts to spuder. If I turn the jeep off then on again it works fine, for another 20 min then again starts to spuder and I hear backfire from the throttle body. What can it be.

 

Check this out:

 

You still have a leak in the vacuum line to the HVAC system. Most likely place I've found is underneath the coolant pressure bottle shelf within 6-8" of where it enters the firewall. Feel along the plastic line, and it should feel smooth. Any roughness should be considered your leak. Is usually a little easier to check if you lift the bottle and remove the shelf (just two screws). The leak will gradually get worse until you eventually have enough vacuum loss to disable your air vents altogether. Just cut out the bad area and use an appropriate diameter piece of vacuum hose.

 

The jeep starts to loose power too. But if a pull over turn the jeep off, then start it up again it is good for another while. Feels like fuel regulator looses vacuum too.

 

The fuel pressure regulator actually makes less pressure with higher vacuum. That ain't it! Your vacuum ball does not look exactly like the one in Cruisers diagram, but that's academic. It's behind the right front bumper, (round and black). There is a large line, comes off rear of the TB, back to the firewall, across then forward all the way down to that. Then there is a small tube that comes back up to operate your vents, and cruse if you have that. I'm not sure where the check valve is on the large line...maybe right at the ball.

For starters I'd check the ball, see that the lines are on it, that, it's even still there, and it's not cracked.

" if a pull over turn the jeep off, then start it up again it is good for another while"

And this>>>

Anyone with an 87 to 90 Renix Jeep has two or three minutes to see that their
CPS is putting out at least .35 AC volts. I don't mean to come off rude, but if you can't connect a meter and turn the key, we may not be much help.
AC voltage CPS test; Small clips on the meter leads really helps. (you need contact with those little pins while it cranks)

Unplug the connector at the back of the manifold with wires going down to the bell-housing/cps. (it's a three wire plug, with only two wires)
Probe the two wires to the cps with the digital meter set on AC volts, a 200 scale on mine. Yours meter may have a lower 2 volt, or “auto range”.
Crank the engine and note the voltage. (jump it if your battery is low). .35 is minimum. >4 to .8 is more like it.