Unclear about what those fuel trim values you report mean. I presume your truck is OBD II, right? When folks here talk about fuel trim under OBD II, the units of fuel trim are usually %, and somewhere near 0% is normal; something in the range of -2% to 2 % is what you'd like to see ideally for fuel trim. Are you able to somehow convert your FT numbers to the more common %?
From what I can tell, here's how to set the timing. Verify this conforms to the label in the engine compartment. With key off disconnect PCM set-timing connector, tan/ black wire; connect inductive timing light to # 1; run engine, trans neutral, verify 0°BTDC on balancer/pulley. Makes sense to check this with a warm engine.
It's normal on engines with electronic timing for the mark to jump around in normal operation; if the timing mark jumps around when setting the base timing tho, that isn't normal. hmmm .. well, timing is related to the MAP sensor and coolant temp sensor readings, so there could be something wrong there. If you run at 2000 rpm, does the timing change if you disconnect the set-timing connector? It should. Does the timing advance noticeably when you bump the accelerator briefly from idle? It should.
Problems with the crankshaft or camshaft position sensing function could cause a timing problem. How is your engine configured for this function? If those functions are done inside the distributor, there's a pickup coil inside the distributor that might be defective. Or the distributor shaft may be loose in its bushing. That could cause the magnet to dither from the pickup coil. Or maybe the coil spacing from the magnet isn't adjusted correctly.
You may need to replace the distributor to solve all this. But take a look inside and outside, including the cap, make sure there are no cracks, no signs of spark caused carbon "tracks", the rotor looks ok, and the plug wires are in good shape. Are the wires connected to the spark plug in order?: I think for your engine it is 165432
The timing chain affects the valve timing, normally not the ignition timing.
My guess, you need a new distributor, or have the existing one rebuilt.
The large advance the shop dialed in just to get it to run could mean there's a problem with the crankshaft pulley position or timing marks too. The gadget that produces the sparks should be considered suspect also, called ESC I think. Do you see a good healthy blue-white spark at the spark plug electrodes?