new here hoping for some advice

[New kid on the block]

Hi guys i have a 1987 heavy half silverado that my brother and i started to work on and we recently ran into a problem that ive not heard of before. When we got the truck it need an entire clutch, slave, starter and flywheel. I got the truck from my dad who sent it to a shop to have the clutch and stuff changed and after he got it back lost interest so i bought it to hopefully be able to drive. The list of stuff done is too long to mention but the issue were having currently i have no idea where to start to diagnose it. The truck wouldnt drive at times and we thought it was the clutch as it would start to drive and then "disengage" turned out to be the pinion gear seal for the rear end and we changed it and it got better then the map sensor went changed that and drove less than a 1/4 of a mile before it started grinding really heavily in the front pulled it back and then started the truck and it will grind in/out of gear in/out of 4x4 with the clutch engaged and with it disengaged the only time it wont grind is when the T-case is in neutral..... is the T-case the issue? If so would the t-case from an 83 heavy half chevy work? Sorry its so long