By they way your describing your problem , it sounds as though the t-case is in neutral and may be stuck there. Be real careful hitting park on the transmission while the t-case is in neutral since you can cause damage to the park mechanism or even reverse. On a manual trans , you've got the clutch to partially engage the main trans to help nudge the t-case carefully into 4hi but as soon as you cross neutral on the t-case is when the grinding begins. This is when you got to know when to apply torque and when to stop applying it. It's really like double clutching a non-synchromesh transmission . You need to match engine speed , gear ratio and road speed. The 4LO is a granny gear and needs to be engaged practically standing still but need a little nudge to help mesh the teeth of the gears together. With an auto trans , you'll want neutral on the main trans as your crossing out of or into 4LO. 4HI is easy to hit but crossing neutral to 4LO takes some practice or it growls at you. If you are repeatedly getting stuck in neutral and can't hit 2HI or 4HI from neutral , you may want to kill the engine to prevent over torque and causing damage . Not saying this is normal operation , and it's not but try this and carefully move the t-case lever to next range from neutral to 2 or 4 HI to make certain the t-case shifts and bears are working. Don't want to say that you got a problem with the detent spring loaded ball in the shift mechanism so we want to make sure the the gears work ok in the t-case . Once back in 2HI , then try it again and see if you can engage 4HI , through neutral and to 4LO . I've had the Dana 20 , NP203 and now a Dana 300 . They all behave similar . Just my NP242 which is syncromesh ( shift on the fly) dosen't have that issue. First see if over torque is holding the t-case in 4LO or if you've really got a problem. Hey , how is the fluid level in the t-case ? Without enough lube , too much friction and gears won't physically slip on or off each other.
Yeah , can you tell us if it's got the original trans / t-case or have they been swapped out ?
Once had a similar situation with an old '78 jeep Cherokee . 4spd and 2spd t-case. Locking hubs. Had to climb a hill on a muddy slope in pitch black morning on opening day once and had four guys hanging off the rear tail gate . Couldn't back up down a hill without causing a safety issue to anyone so I couldn't shift out of 4LO . T-case was was over torqued. Had to try 2nd gear but still spun the tires. Once they got off , I backed down enough to get through neutral on the t-case with main trans back in 1st . You don't want to shift t-case in reverse . The gears will spin backward when engaged but not meant to shift in reverse motion. Gears aren't cut that way.
Lets make a long story short. Engine off , get the t-case back to 2wheel and try again . See if you got all forward speeds in 4wheel (Lo & hi range). If the t-case is in neutral and won't engage Lo or hi , don't shift the main trans into reverse or drive. That's abuse or damaging. You've got to get the t-case back into gear before getting main trans into gear too. The same for a manual trams . Check the lube level and try again . Let us know how it went.
