1987 4x4 overdrive and 4wd not working

[the walrus]

My 1987 1/2 ton 4x4 TBI 305 TH700-R4 (completely stock) is not shifting into overdrive. The current extents of my troubleshooting have been limited to checking the fluid which is at the proper level. O.d. was engaging just fine a few hundered miles ago with no major events happening to the truck in the meantime. I'm pretty sure that it has a lock-up torque convertor which I can feel engaging in third gear after about 40mph under steady load but no o.d.
Any tips on where to start looking????

Also, on an unrelated note (I think) the front wheels are not being driven with 4wd engaged. Front hubs are manual warns and if I lock the hub, raise and spin the wheel the drive shaft turns with the hubs locked and idles with the hubs not locked, so I'm fairly confident that they are working correctly. 4x4 low ratio enages as it should, but only drives the rear wheels. Whats the first thing to look for???

thanks in advance.