Now I am not a transmission guy, but it sounds to me like the syncros are gone in the tranny. I had a very similar problem with my 48 Willys about 10 years ago. It kicks out of gear when you have a load on it. Does it grind when you shift at all? If it does, the clutch might need adjusting. But another thing is when was this thing last rebuilt? It might just need a good going through.
As for swapping out to a 5 or 6 speed, they are nice if you don't wheel it, but they don't have the same low end gearing that an NP does. I know of a site that has a gear calc. on it and any way you look at it, the NP has a lower gear ratio than anything else out there.
https://home.comcast.net/~jcmiley/bronco%20gears.html
Take my rig for example. I have an NP 435, T-shift xfer, 4.11 gears & 33" tires. Here are a few gearing examples with what I could have vs. the NP.
NP Ratio: 67.47:1
Stock 3 speed: 30.15:1
T-18: 63.74:1
ZF: 57.69:1
NV4500 (w/6.32 1st gear): 63.74
Even with the low gears in an NV I am still about 5% lower geared than that is. So for an arguement for offroad & keeping the NP, I think that about says it. But if the thing is giving you nothing but headaches, it doesn't hurt to have a look at a different tranny.
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