I have to bring the rpms up to 2500 then let off of the throttle to get it to shift into 2nd gear. The transmission has been rebuilt. What would make it do that. It's a 2004 blazer 4x4

bring it back

No not the whole transmission. Could be a band or servo. It still has to come out and be tore down to fix.

That could be a perfect reason to take it back to the shop where it was just repaired for repeat service. Either that or you spun out in the mud.

For car not changing gears it can be shifter assembly issue, or it can also be transmission issue. Get the shifter assembly inspected with ignition OFF.

For more troubleshooting, I suggest you to get the Following help link checked, its complete step by step procedure to inspect gear shift issues: -----

GEAR SHIFT ISSUES

http://alltypesofpartsreplacing.blogspot.com/2012/11/gear-shift-issues.html

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These will help.
Thanks.

I have a 92 Crown Vic first and second works fine reverse works fine when it shifts in the third gear it goes to slipping and overdraft can you help me fix this problem thank you

HI did you replace the epc = electronic pressure control solenoid

The speed sensor on the transmission along with the throttle position sensor on the engine deliver the signals to the transmission computer dtermining shift timing, the throttle position sensor is probaly out of wack, there should be a stored trouble code however this transmission code can only be read by a transmission shop or the dealership, I guess you could change out the TPS sensor easy enough however this is merley an educated guess on my part to be sure have the computer checked for stored trouble codes.

you have to adjust the gear lever in park untill the gearbox is set precicely in park by tweaking untill just right.
Hope this helps!

The possiblities that come to mind are
1 Bad synchro in transmission 2 bad bearing(s) in transmission input shaft.   3 Bent shift linkage or shift fork (I doubt this, but it's possible)
Only number 3 is relatively cheap.  I'd go back to where/who you bought the car from, if it was that way when you bought it, and ask them to help.  It can't hurt.

Using force is NOT the solution.  If the syncros are bad, until you get them replaced, you'll be better off to determine the RPM difference between 2&3 gears (maybe 800-1000?) and also 4&5 gears (maybe 500-800?) and learn to put a modest amount of effort at exactly the time the rpms fall into that diffference.  
Example Say the difference between 2nd gear at 3500 RPM and 3rd gear is 1000 RPM, then when you take it out of 2nd for an upshift to third,  delay until the RPMs are just getting to 2500. If you are on a budget and can't fix the transmission (it's going to be big bucks) you may find you can get pretty good at this.    It is possibly to shift a car this way without even using the clutch, although I don't recommend this and it takes a great deal of practice and luck.
While you have this problem, don't downshift into 3rd,  just run the RPMs down in 4th (say to 2000 or less) until you can shift into 2nd at an acceptable RPM. The least amount of shifting you can do until you resolve this the better.
Another choice until you can fix it, is to just run up the RPMs in 2nd and then shift to 4th, and then forget you have a fifth gear.  This is much better than forcing, it's possible to do more damage by forcing. 
Is it hard to get into 3rd from 4th AND 2nd?  
..ajm

The trans module put the trans into limp in (2nd gear) any time it detects a problem. It could be electrical or mechanical. You'll have to have some put a scanner on it to check it out.