You just get under the truck on the passenger side, there should be 2 rods coming from the bottom of your t-case shifter to the t-case itself. Just look at them and make sure they are attached right. They are held on with cotter pins and washers and can rust away letting one of the rods come loose. I'll have to check the 203 I have, there may only be 1 rod, I'm not sure. But the little zig zag plate on yours, I dont know if that belongs. 203s should have 4-Hi. Neutral, 4-Lo and 4-Lo Loc. (There may be a 4-Hi Loc too) After reading the other posts you said it moved a little in 4-Lo, I'm thinking like HRPC, 4-Lo makes so much torque from gear reduction it helped it move a little. I'm starting to believe your problem lies with the transmission. Either a dead torque converter or burnt clutches.
The NP203 Has 5 locations for the shifter, Starting in the position closest to the seat it should be:
4 High Lock
4 High
Neutral
4 Low
4 Low Lock.
The tail housing is aluminuim but the rest of the case is Cast.
The NP205 has 4 shifter locations. Starting from the position closest to the seat it should be:
4 High
2 High
Neutral
4 Low
The 205 is easilly recognizable by its Heavy cast iron case, There is also a small circular plate the bolts to the rear of the case Aprox. 2-2.5" in diameter, It will have 3 Bolts that hold it to the case. The shifter handle on the 205 is just round.
The shifters are Different aswell, The 205 has a center pivot bolt in which the shifter pivots on.
The 203 has 3-4 linkage rods, and the shifter itself is actually a side rail shifter style. usually the handle is like flit iron that tapers back down to a round shaft for the nob It is also usually chrome plated.
like C4 said the zig zag plate is NOT original to the truck,
As for your transmission, You need to check the line pressure, Sounds to me like the trans is smoked.
