My 1991 jeep cherokee 4x4 with no warning started not going into gear when you try putting in gear it grinds real bad if you work with it enough it will go into is sometimes any ideas

driving on what road.
not on pavement in low. (read the operators guide on low)?
its for dirt or ice.
not pavement.
and will damage parts. if you ignore that.
put car on dirt, and test now, working now, is it?
sure.

Try neutral. or in drive, but just crawling. Give light pressure to the stick and sort of let it just fall into 4x4

I had a ford explorer do the exactly same thing. the snow wedge the 4x4 linkage on the transfer case. i had to get underneith with a wrench and simply turn it until it engadgd... goodluck

It sounds to be like you have a syncronizer out on your transmission. This helps your gears mesh without grinding.

I've done several of these swaps that sound good at the time but end up being a Huge problem.I would start with the correct harness for the engine you've put in.It must include the under hood harness and what ever harness connects to it thru the firewall to the computer.You'll also need everything connected to the transmission to the computer including any thing connected with the brake and accelerator pedals.Next get the wiring diagrams from Jeep or buy the Chilton or Haynes manual,they have the diagrams included.Get the harnesses connected to the engine,trans and computer,then work backward towards hooking up everything else.I really feel your pain,I've put 20 year newer engines in older cars,V-8's where 6's were,an olds V-8 where a in-line six was(ford),Late model v-8 in a 59' Chevy.All these were simple compared to what your going thru.Please contact me if I can be of any help?

Try to clean with brake cleaner and put jb weld for a temporary fix.

have you tried a rolling shift? leave trans in drive and try moving 4x4 lever. it works on mine!

Just pull up on the 4 wheel drive lever .. You can shift on the fly to part time and full time but you gotta stop and in park or neutral you can shift into 4 wheel lo .. I have one myself .. love those jeeps .. you "understand"

The part time switch in the gearbox is sticking in the on position locking the 4x4 on. It feels and looks really bad because the front wheels are trying to pull the car left or right, while the rear axle is pushing you forward. This is at it's worst when in full lock on road or hard surfaces. This is why 4x4 is for off road only.

The first thing you want to check is the clutch master fluid, make sure there is enough in there. This sounds like there is air in the system.
You will have to bleed out the clutch slave cylinder. Get yourself a manual for the car and it will walk you through it.