My 2000 GMC Sierra 1500 will only drive in 1&2 but not 3&D wuts up with that??? And also...can any chvy ten bolt rear end work on my truck?? ive got a blown rear end and need to know what applicatipns i can use or if any type of swap is possible...also can i put posi on my 2wd

No 3 and 4 is probably 3-4 Clutch Pack. If so, it needs to come out for a rebuild/replace.

//2000CAYukon

another problem i have is the rear end is blown....does the gear ratio i use matter..? or can i throw a 10 bolt rear end on and be fine....i found a ten bolt rear end but it has drum brakes and mine has disc...interchangable or not?? i dont know what type of rear end...Non Posi is all i know... Anyone have an idea of what type of rear end i have..... and what other rear ends i can use...i want posi if possible

2WD 2000 GMC Sierra 1500 SLE
Eextended Cab 5.3L V8 Vortech

Most auto wreckers will have a book that lists which parts fit which years and models. Might be a good place to start.

Gear ratio does matter. It can have an affect on mileage and performance.

//2000CAYukon