4x4 wont engage - 1991 GMC Sierra

Mechanical engineer from Ford can help with that regards.

check that the hub locks are engaging and that the selector actuator is actually putting it in 4wd.

check that the hubs are locked in correctly (both sides)

Nine time`s out of ten it`s a fault on the accuator diaphargm,try changing it...

There is an acuator in the front diff the are common to quite in late 80 to early 90 Chevrolet's / GMC

Could be a problem in either the 4wd controller or the actuator on the transfer case. If you plan to keep the vehicle for awhile, purchase a used service GM manuals for your specific vehicle. The diagnostic flow charts are extremely helpful.

Could be a transfer case shift solenoid or the relay that activates the solenoid.

go to the front axle and take the actaushwaiter out and gert a new one

It is possible that the vacuum actuator that engages the 4X4 is faulty. It will have to be tested with vacuum pressure tester.
This is usually the case when 4X4 doesn't engage.
Other case is the module inside tranny is defective and may need rebuild

thier is a shift fork in the front differential that engages the front axles. this is commonly broke.