Ben, I will attempt not to write a book about GST, so I'll give you the short story; Inside the trans you have a hydraulically engaged spring released clutch pack. On the left side of the trans just below the floor board you have the GST valve assy. All transmission shifts are done with hyd shift pistons located in this valve and connected to a mechanical shifter. With the exception of fwd/rev shuttle. Fwd and reverse is shifted manually, however, there is a "release pin" in the valve that the shuttle shaft depresses while in transition from fwd to rev and rev to fwd that dumps clutch pack pressure and disengages the hyd clutch untill the fwd or rev gears are engaged. Now, you have four gear sets in the trans, fwd of the clutch pack, aft of the pack you have the hi-lo gear set and the fwd-rev gear set all of which are synchronized. In gears 1-4 the hi-lo set is in low range, in gears 5-8 the hi-lo set is in high range, hence 8 speeds in fwd and rev. The gear shifter itself is cable operated and runs down to the GST valve. The cables have a tension adjustment. The cables move a rotary valve that ports fluid through a manifold that determines what shift pistons to port fluid to, to shift the trans to what gear you select, at the same time, during the shift, clutch pressure is dumped to provide a smooth clean shift. After the shift is made clutch pressure rises and engages the clutch pack. The shift is made and you are on your way. So, when making a shift from 0 to 1, You move the shifter, the cable moves the rotary valve which ports fluid to the 1-2/5-6 shift piston and dumps pack pressure, pressure builds in the shift piston, moves the shifter and engages 1st gear, clutch pressure buids back up through an accumulator in the valve assy, which allows pressure to build gradually and engage the clutch pack. When going from 2-3 the 1-2/5-6 piston shifts to neutral and the 3-4/7-8 shift piston shifts the gear set to 3rd gear. Same process to fourth. Now when going from 4-5th gear, the 1-2/5-6 piston shifts back to the 1/5 position, the 3-4/7-8 piston shifts to neuatral. At the same time, the hi-lo set shifts to hi range, thus the longer pause going from 4-5th gear. O.K., still with me? In the lower gears 1-4, pack pressure needs to be brought up more gradually to prevent hard aggressive shifting, but in the higher gears, pack pressure needs to be brought up quicker to prevent too much lag between shifts, so a low pass and a high pass valve in the GST valve assy compensates for this difference.
Why did I explain all this? Because what you are describing sounds like the clutch pack may not be disengaging completely between shifts. A restriction (foreign object) in the GST valve assy, warped clutch discs or an obstruction in the pack may be causing this. ALL the synchronizers in the trans being bad sounds unlikely, but not impossible.