Highway driving won't hold 75 in 5th gear

I can run 85 + in 4th gear all the time no problems but when I put it in 5th it doesn't want to hold 75 no matter how much throttle I give it

 

Update the Jeep is a 89 Cherokee Laredo 4x4 5 speed with 4.0 inline 6 not 100% sure on gear ratio in rear diff

 

Size matters if the tires are oversized from stock. Big tires look great and offer better off-roading but can affect upper highway speeds especially in top gear. Your jeep should be over the initial start/stall issues otherwise make sure all systems check out. All the 1989 "tuneup" issues may have a role.

 

What aggrex said. If you have significantly bigger tires over stock like 33s you'll have a hard time maintaining highway speeds in 5th. To get overdrive back you'll need to rehear your axles to something like 4.10s.

 

Yeah these are what I'm running but nothing crazy

 

Do you know what gears you have now? If you are really having trouble holding 5th I bet you have something like a 3.07. 4.10s are perfect for 33s. If you plan on going bigger than 33s I would wait to regear until you reach the size tire you are aspiring to get. I may be mistaken, but I believe there are some rear Dana44s with 410s that came in Cherokees with a towing package. It would be way cheaper to find someone's used axles (Front and REar) already geared and swap them in than it would be to regear to 4.10s, that is if you can swap the axles yourself and save the 10 hours labor they will charge you for the swap. A regear should run you around $1400. You can usually find a set of decent axles for under $5oo for a XJ.

 

OD is almost a 25% reduction in torque, with high gearing and big tires it's just too much torque loss to maintain speed.
If you could find someone who swapped axles out of an Auto XJ the 3.55 gears would get you in the ballpark of stock performance.