I can only get my 91 gmc truck to go into fourth gear or neutral gear

THAT IS LIMPHOME, STUCK IN1 GEAR.
that second act it is called FLARE (OR FALSE NEUTRALS)

1. check all fuses.
2. scan the PCM see DTC errors ,bingo
3. or get it scanned. in shop.

91 is old, most that old are bad. box.
GM renamed the 700R4 to 4L60

it is not a 4L60E , not electonics shifted nor any kind of TCM

it is a pure Hyrdamatic box , a kinda Hyraulic brain side,
there are 2 causes. or more.
low pump pressure or its regulator bad.
leaks in all seals inside adding up to way too much
valves stuck inside.
jammed TV cable, make tranny thing passing gear full time.
and more lots more buy the 4L60 book and read it. from
ASTG $35

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313&_nkw=manual%3B+ASTM+4L60+manual&_sacat=0


no better books exists unless by by CLIFF RUGGLES

I have both.

https://www.ebay.com/itm/384149612942?epid=1230034643&hash=item597119e98e:g:Y54AAOSwQctbPmPZ&amdata=enc%3AAQAHAAAAsEoKQu0eiwGgLMD62cXMnfW1%2BNeVLroeICaW0pkzGm2ntaAb0JtAb%2Fdk9LIHN6z06%2BAcXFoU%2FFW3qQ7Gq6WT6%2FFksX67MaMsDb4HEl6tNMR3Ldqzw3csf6PgxM0lZtlcpN0KAun%2B5tz32FYyO3cTEkPJgDDtg%2FdsvATFvd08airyN4aFurj8QRozilFQDgXyrLONMBZaeHi1ehR1E%2Fg4jR3fZ7HmccbeGCciO%2FUOSBus%7Ctkp%3ABk9SR-68zLjkYA


cliff book is one of the best reads on topic I have ever seen
and I read them all but only few exists . chiltons is junk printed on junk yellow news paper rag crumbles up.

Yes you have..try again.

could be worn universal joint bearing if noise coming from rear end of vechicle.

check the oil level of the gearbox first

SHIFT LINKAGE WORN OR DAMAGED.BROKEN OR LOOSE INPUT GEAR BEARING RETAINER, COULD HAVE DAMAGED OR WORN CHECK BALLS FORK ROD BALL GROOVES OR CHECK SPRINGS, WORN MAINSHAFT OR COUNTERSHAFT BEARINGS,WORN SYNCHRONIZERS OR CHIPPED OR WORN GEAR TEETH. MANY THINGS WILL CAUSE MANUAL TRANSMISSION TO JUMP OUT OF GEAR.

Sounds like the actuator that activates the 4 wheel in the transfer case went from 4 to neutral, and it will not go back into gear. You climg under the truck on the drivers side. Look for a electrical part bolted to the side of your transfer case( back end of trans were drivlines hook up) and give it a very gentle rapp with a rubber malat while somone is activating the switch back and fourth into 2 wheel. This may get you going till it can get fixed. It may be the button itself.

Try going to your local parts store (such as AutoZone) and ask about the transmission control solenoid. Go here for the direct link to their website with that specific part for your 91 maxima.

Check transmission lock up selenoid.

You need to remove top off gearbox and with a pry bar or screw driver pull the selector back into neutral Your gearlever has popped out of selector Gearbox does not need to be removed

Check the fuel filter-they had a recall on replacement filters(fram?-just google GMC Recalls) or go to 91 GMC Sonoma Repair=many sites to tell you and download manuel. Then the library has books if you dont. Good luck and God bless1
Scoyboy